This would work, if only Parallels actually supported USB booting in the boot device menu. (They do not.)
Apparently it has been promised for many years, so I won't hold my breath. But for the techs and engineers reading these forums, please move it to the top of the list. With Apple releasing more and more optical drive-less machines, the ability to install Windows from a USB flash drive is all the more vital moving forward.
Thanks guys!