I tested the proposed package and it works as expected.

root@t-unsubsidized-piper:~# lsb_release -rd
Description:	Ubuntu 14.04.5 LTS
Release:	14.04

root@t-unsubsidized-piper:~# uname -r
3.13.0-137-generic

First, I confirmed it still fails on 3.12-1ubuntu0.1

root@t-unsubsidized-piper:~# truncate -s 1G testloop.img
root@t-unsubsidized-piper:~# losetup /dev/loop0 testloop.img
root@t-unsubsidized-piper:~# mkfs.ext4 /dev/loop0
mke2fs 1.42.9 (4-Feb-2014)
Discarding device blocks: done                            
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
65536 inodes, 262144 blocks
13107 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=268435456
8 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks: 
	32768, 98304, 163840, 229376

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done

root@t-unsubsidized-piper:~# mount /dev/loop0 /mnt
root@t-unsubsidized-piper:~# rm testloop.img
root@t-unsubsidized-piper:~# mkfs.btrfs --force /dev/vdb
Error: error checking /dev/vdb mount status
root@t-unsubsidized-piper:~# echo $?
1

Next I added trusty-proposed and updated

root@t-unsubsidized-piper:~# apt-cache policy btrfs-tools
btrfs-tools:
  Installed: 3.12-1ubuntu0.1
  Candidate: 3.12-1ubuntu0.2
  Version table:
     3.12-1ubuntu0.2 0
        500 http://nova.clouds.archive.ubuntu.com/ubuntu/ trusty-proposed/main amd64 Packages
 *** 3.12-1ubuntu0.1 0
        500 http://nova.clouds.archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     3.12-1 0
        500 http://nova.clouds.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

After installing the upgrade, I repeated the mkfs.btrfs command, but this time it succeeds

# sudo apt install btrfs-tools
sudo: unable to resolve host t-unsubsidized-piper
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be upgraded:
  btrfs-tools
1 upgraded, 0 newly installed, 0 to remove and 7 not upgraded.
Need to get 332 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://nova.clouds.archive.ubuntu.com/ubuntu/ trusty-proposed/main btrfs-tools amd64 3.12-1ubuntu0.2 [332 kB]
Fetched 332 kB in 0s (827 kB/s) 
(Reading database ... 51345 files and directories currently installed.)
Preparing to unpack .../btrfs-tools_3.12-1ubuntu0.2_amd64.deb ...
Unpacking btrfs-tools (3.12-1ubuntu0.2) over (3.12-1ubuntu0.1) ...
Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
Setting up btrfs-tools (3.12-1ubuntu0.2) ...
update-initramfs: deferring update (trigger activated)
Processing triggers for initramfs-tools (0.103ubuntu4.10) ...
update-initramfs: Generating /boot/initrd.img-3.13.0-137-generic
Processing triggers for libc-bin (2.19-0ubuntu6.13) ...
root@t-unsubsidized-piper:~# mkfs.btrfs --force /dev/vdb

WARNING! - Btrfs v3.12 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

Turning ON incompat feature 'extref': increased hardlink limit per file to 65536
fs created label (null) on /dev/vdb
	nodesize 16384 leafsize 16384 sectorsize 4096 size 40.00GiB
Btrfs v3.12
root@t-unsubsidized-piper:~# echo $?
0