From imgae #17, the UI stayed at the spinning 'checking for updates' for as long as I could wait (at least 10 minutes), so I went with:

# adb shell system-image-cli -c trusty-proposed -b 0 -v
[systemimage] Nov 13 10:47:19 2013 (5513) running state machine [trusty-proposed/mako]
[systemimage] Nov 13 10:47:19 2013 (5513) Looking for blacklist: https://system-image.ubuntu.com/gpg/blacklist.tar.xz
[systemimage] Nov 13 10:47:20 2013 (5513) Requesting group download:
	https://system-image.ubuntu.com/gpg/blacklist.tar.xz -> /var/lib/system-image/keyring.tar.xz
	https://system-image.ubuntu.com/gpg/blacklist.tar.xz.asc -> /var/lib/system-image/keyring.tar.xz.asc

[systemimage] Nov 13 10:47:20 2013 (5513) Running group download reactor
[systemimage] Nov 13 10:47:31 2013 (5513) Group download reactor done
[systemimage] Nov 13 10:47:31 2013 (5513) Local blacklist file: /var/lib/system-image/blacklist.tar.xz
[systemimage] Nov 13 10:47:31 2013 (5513) Looking for: https://system-image.ubuntu.com/channels.json
[systemimage] Nov 13 10:47:31 2013 (5513) Requesting group download:
	https://system-image.ubuntu.com/channels.json -> /tmp/system-image-z9hrkv/channels.json
	https://system-image.ubuntu.com/channels.json.asc -> /tmp/system-image-z9hrkv/channels.json.asc

[systemimage] Nov 13 10:47:31 2013 (5513) Running group download reactor
[systemimage] Nov 13 10:47:32 2013 (5513) Group download reactor done
[systemimage] Nov 13 10:47:32 2013 (5513) Local channels file: /tmp/system-image-z9hrkv/channels.json
[systemimage] Nov 13 10:47:32 2013 (5513) got channel: trusty-proposed
[systemimage] Nov 13 10:47:32 2013 (5513) found channel/device entry: trusty-proposed/mako
[systemimage] Nov 13 10:47:32 2013 (5513) Requesting group download:
	https://system-image.ubuntu.com/trusty-proposed/mako/index.json -> /tmp/system-image-z9hrkv/index.json
	https://system-image.ubuntu.com/trusty-proposed/mako/index.json.asc -> /tmp/system-image-z9hrkv/index.json.asc

[systemimage] Nov 13 10:47:32 2013 (5513) Running group download reactor
[systemimage] Nov 13 10:47:32 2013 (5513) Group download reactor done
[systemimage] Nov 13 10:47:33 2013 (5513) Upgrade path is 20
[systemimage] Nov 13 10:47:33 2013 (5513) Requesting group download:
	http://system-image.ubuntu.com/pool/ubuntu-918c4fd296961d2c273658540f8072cfe03d57d5fdc97b750073f7daa0149987.tar.xz -> /android/cache/recovery/ubuntu-918c4fd296961d2c273658540f8072cfe03d57d5fdc97b750073f7daa0149987.tar.xz
	http://system-image.ubuntu.com/pool/ubuntu-918c4fd296961d2c273658540f8072cfe03d57d5fdc97b750073f7daa0149987.tar.xz.asc -> /android/cache/recovery/ubuntu-918c4fd296961d2c273658540f8072cfe03d57d5fdc97b750073f7daa0149987.tar.xz.asc
	http://system-image.ubuntu.com/pool/device-0e42c8ad6c2174c9f3fa4a580a620e57b9df4b3c0aed54b425f4e2990ad4f6f7.tar.xz -> /android/cache/recovery/device-0e42c8ad6c2174c9f3fa4a580a620e57b9df4b3c0aed54b425f4e2990ad4f6f7.tar.xz
	http://system-image.ubuntu.com/pool/device-0e42c8ad6c2174c9f3fa4a580a620e57b9df4b3c0aed54b425f4e2990ad4f6f7.tar.xz.asc -> /android/cache/recovery/device-0e42c8ad6c2174c9f3fa4a580a620e57b9df4b3c0aed54b425f4e2990ad4f6f7.tar.xz.asc
	http://system-image.ubuntu.com/trusty-proposed/mako/version-20.tar.xz -> /android/cache/recovery/version-20.tar.xz
	http://system-image.ubuntu.com/trusty-proposed/mako/version-20.tar.xz.asc -> /android/cache/recovery/version-20.tar.xz.asc

[systemimage] Nov 13 10:47:33 2013 (5513) Running group download reactor
[systemimage] Nov 13 10:54:17 2013 (5513) Group download reactor done
[systemimage] Nov 13 10:54:31 2013 (5513) all files available in /android/cache/recovery
[systemimage] Nov 13 10:54:31 2013 (5513) preparing to reboot
[systemimage] Nov 13 10:54:31 2013 (5513) rebooting

Note that the 7 minutes without feedback almost triggered a "bah, broken somehow, let's reboot" which get delayed because I was typing this comment ;)