I have shown this at presentations several times now; Hyper-V Server 2008 R2 operating from UFD. People often asked for a copy of the drive or a how-to-guide to make this work.
Although totally unsupported, you can now configure a UFD with Hyper-V Server 2008 R2 yourself. See here for full details.
If you don’t want to go through all manual steps to make this work, you can get a tool which does it all for you. You can find that tool here.