I use a MacBook Air 2011 as my everyday computer, including a little bit of CAD work in SolidWorks 2011.
I work in Windows 7 95% of the time and run a tiny Mac OS X partition and a large Boot Camp Windows partition on the internal SSD. The remaining 5% of the time I'm in Ubuntu Linux, which I boot off a fast USB 2.0 Flash drive.
To be honest, I've never booted a full Windows off an external flash, but I have booted Windows PE (Hiren's Boot CD/USB).
Here are my thoughts on what you're trying to do:
0) I wouldn't do a VM approach. I've seen Parallels running Windows + SolidWorks and was not happy with the results at all. Slow, unresponsive, and getting hardware graphics acceleration working properly was a huge pain. Even with acceleration it working, it felt barely usable.
1) Not all USB sticks are created equal. I'm assuming the Macs you are using are USB 2.0 (not 3.0... argh Apple). There is a huge variation in USB 2.0 flash drives in terms of speed. If you bought the cheapest no-name ones that happened to be on sale, chances are you won't be happy with your result. Check out:
http://usbflashspeed.com/ for reference. I wouldn't try running an OS off anything less than 20MB/s read and write.
2) Install rEFIt (
http://refit.sourceforge.net/) on all the Macs, as it will make OS selection on boot-up much easier. rEFIt seems to be a lot less picky when it comes to booting non-Mac partitions, compared to the standard Mac boot.
3) I don't know how Windows Activation (and drivers) will work. I'm assuming all the Macs you are going to use have identical hardware? Once you activate Windows with a specific USB flash drive, you may need to use that computer/flash drive combination forever. You might not be able to interchange flash drives between computers.
4) Create an exFAT partition on the local hard drive for virtual memory, or disable virtual memory altogether. Using flash for virtual memory is not a great idea since it has a limited number of read/write cycles before it will fail. Enabling ReadyBoost is an interesting option, but I don't know if it will help if you run it on the same USB Flash drive as your OS. If you want to try it, create an extra empty exFAT partition on the flash drive.
5) I would really stay away from Windows Server if possible. Driver support always seem to be an issue with Server versions, and there is a lot of bloat you will never use. Plus it will be tuned to run background tasks with a higher priority, meaning it won't feel as responsive.
What you are trying to do really has a good use case, regardless of whether you are using a Mac or PC.
It'd be nice for teams to be able to have a stash of USB flash drives that they could send home with students and mentors, and have an instant CAD environment all ready to go.
Please keep us updated!