Tried again with the latest 1.9 bits from maas/proposed.

Attempted a RAID0 once more after zeroing the two disks manually, then attempting to deploy.  This is the install log from the maas UI:

Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Error: /dev/sda: unrecognised disk label
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['mdadm', '--assemble', '--scan'] with allowed return codes [0, 1, 2] (shell=False, capture=False)
mdadm: No arrays found in config file or automatically
clear_holders running on '/sys/block/sda', with holders '[]'
Running command ['sgdisk', '--zap-all', '/dev/sda'] with allowed return codes [0, 1, 2, 5] (shell=False, capture=True)
labeling device: '/dev/sda' with 'msdos' partition table
Running command ['parted', '/dev/sda', '--script', 'mklabel', 'msdos'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['blkid', '-o', 'export', '/dev/sda'] with allowed return codes [0, 2] (shell=False, capture=True)
Can't find a uuid for volume: sda. Skipping dname.
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Error: /dev/sdb: unrecognised disk label
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['mdadm', '--assemble', '--scan'] with allowed return codes [0, 1, 2] (shell=False, capture=False)
mdadm: No arrays found in config file or automatically
clear_holders running on '/sys/block/sdb', with holders '[]'
Running command ['sgdisk', '--zap-all', '/dev/sdb'] with allowed return codes [0, 1, 2, 5] (shell=False, capture=True)
labeling device: '/dev/sdb' with 'gpt' partition table
Running command ['sgdisk', '--clear', '/dev/sdb'] with allowed return codes [0] (shell=False, capture=False)
Creating new GPT entries.
The operation has completed successfully.
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['blkid', '-o', 'export', '/dev/sdb'] with allowed return codes [0, 2] (shell=False, capture=True)
Can't find a uuid for volume: sdb. Skipping dname.
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
adding partition 'sda-part1' to disk 'sda'
Running command ['parted', '/dev/sda', '--script', 'mkpart', 'primary', '2048s', '3907020799s'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['sgdisk', '--zap-all', '/dev/sda1'] with allowed return codes [0, 1, 2, 5] (shell=False, capture=True)
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['blkid', '-o', 'export', '/dev/sda1'] with allowed return codes [0, 2] (shell=False, capture=True)
Can't find a uuid for volume: sda-part1. Skipping dname.
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
adding partition 'sdb-part1' to disk 'sdb'
Running command ['sgdisk', '--new', '1:2048:3907020799', '--typecode=1:8300', '/dev/sdb'] with allowed return codes [0] (shell=False, capture=False)
The operation has completed successfully.
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['sgdisk', '--zap-all', '/dev/sdb1'] with allowed return codes [0, 1, 2, 5] (shell=False, capture=True)
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['blkid', '-o', 'export', '/dev/sdb1'] with allowed return codes [0, 2] (shell=False, capture=True)
Can't find a uuid for volume: sdb-part1. Skipping dname.
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sda'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['partprobe', '/dev/sdb'] with allowed return codes [0, 1] (shell=False, capture=False)
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['mdadm', '--zero-superblock', '/dev/sda1'] with allowed return codes [0] (shell=False, capture=False)
mdadm: Unrecognised md component device - /dev/sda1
Running command ['mdadm', '--zero-superblock', '/dev/sdb1'] with allowed return codes [0] (shell=False, capture=False)
mdadm: Unrecognised md component device - /dev/sdb1
Running command ['udevadm', 'settle'] with allowed return codes [0] (shell=False, capture=False)
Running command ['udevadm', 'control', '--stop-exec-queue'] with allowed return codes [0] (shell=False, capture=False)
Running command mdadm --create /dev/md0 --run --level=0 --raid-devices=2  /dev/sda1 /dev/sdb1 with allowed return codes [0] (shell=True, capture=False)
mdadm: Defaulting to version 1.2 metadata
mdadm: RUN_ARRAY failed: Invalid argument
An error occured handling 'md0': ProcessExecutionError - Unexpected error while running command.
Command: mdadm --create /dev/md0 --run --level=0 --raid-devices=2  /dev/sda1 /dev/sdb1
Exit code: 1
Reason: -
Stdout: ''
Stderr: ''
Traceback (most recent call last):
  File "/curtin/curtin/commands/main.py", line 208, in main
    ret = args.func(args)
  File "curtin/commands/block_meta.py", line 63, in block_meta
    meta_custom(args)
  File "curtin/commands/block_meta.py", line 1106, in meta_custom