Thought I'd pass along you can make this work even though you're not supposed to.
Used a 128 GB LaCie portable Thunderbolt SSD with a 15", Early 2011 MacBook Pro i7.
You do still need a Boot Camp partition on your internal drive, but at least it can be the minimum size, which would be so small as to be unusable if it were actually your C Drive. Also, you still get write privileges on that partition, so I don't see why you can't use it as storage, though obviously, it's got a MS-DOS filesystem. I haven't tried using it for that purpose yet.
The actual process is simple. Format the external Thunderbolt drive with a single MS-DOS (FAT) partition. I called it "BOOTCAMP". Not sure if this particular name is necessary.
From there, you do the install as normal. Start with the Boot Camp Assistant, and use it to create a Boot Camp partition on your internal drive. Minimum size, natch.
Once the actual Windows installation starts, select the Thunderbolt drive as the installation target. Windows 7 wants an NTFS filesystem, so you'll need to reformat the drive by selecting the "Advanced" option and doing it there.
If you used the name "BOOTCAMP" on the Thunderbolt drive, it should be obvious which of the two BOOTCAMP partitions you want to use because one of them is larger than the other.
That's it. Works as normal.
Do not use Boot Camp Assistant to wipe your internal Boot Camp partition. The drivers seem to be squirreled away in there somewhere. Without them, you'll forever be trapped in the BIOS, with a request for a bootable drive.