I’m not sure how this lines up with the EULA, but Steve Makofsky has some interesting pointers about how to run Windows CE of a USB drive.
Really, it’s just a matter of running the Windows CE device emulator, and the “launcher script” that you download actually installs and uninstalls DLLs to the system directory when it’s run, so it’s not quite as portable as the write-ups make it seem.
I was hoping that this would be a “bootable CE on USB” type thing, but it falls well short of that. It’s an interesting novelty just the same.