There are hacks to make it work. However the biggest technical limitation I usually hear of is that supposedly, Windows does not load USB drivers early enough in the boot process for things to work properly, so it results in a crash.
Regardless of that though, it has supposedly been done before, and worked fine. Also, yes, Thunderbolt is more than sufficient to run a full OS off of. In fact since you have a Retina, USB 3 should do as well. And at least with USB you have compatibility across systems should you need to access files on the USB drive on a computer that is not your MBP.
I don't know much about it but I do know you need the Windows 7 install disc ISO. Microsoft provides downloads of it somewhere on their site. If you have an actual disc you can also clone the disc to an ISO image file with Disk Utility.
Once you have your ISO, you do need a running Windows 7 (I think you can also do it with XP) system for the tools you need. One such thing you need is WAIK (Windows Automated Installation Kit) from Microsoft.
I believe WAIK can reconfigure certain Windows files so that booting from USB works, when normally Windows would refuse to if you were to clone a working installation.
Windows is also kind of picky about drive formats unlike other OSes. You have to make a few partitions, make sure a certain one is active, and that they're all NTFS I'm pretty sure. WAIK then handles putting stuff onto each partition I think.
That's just off memory. I Googled real quick and found this video guide:
https://www.youtube.com/watch?v=uvWGV9YJxfQ
It seems to be hit or miss for some people, but it succeeds for most by the look of the comments. You can try it and hope it works for you. If not there's probably a solution anyways.
Be sure to post about any problems you might have or if it succeeds. I'm curious about how this process works in entirety myself as I want to boot Windows 7 off USB for gaming purposes.
