I'm not sure of the feasibility of dual booting from a flash drive, but don't see why it wouldn't work. Honestly, I was thinking of doing something similar myself, just for the sake of trying some different OSes out.
In any event, my purpose for responding to your question is to ask if there is a reason you don't just get a different flash drive to have the different OSes installed on? I mean, they are getting pretty inexpensive right now, and that would allow you more room for each I'm guessing. And we all know what a space hog Windows will become.
Of course, I just went back and read your original post again and realized that the best way to dual boot with windows and Linux is to install windows first then your Linux distro. AFAIK, that's the only way to do it and have a reliably stable windows install. So maybe your best bet is another flash drive.
