I think the filesystem supported naming files PRN, CON, AUX, LPT, COM1, etc. just fine, it's just DOS wouldn't let you "get to" them because it (and by "it" I'm not sure if COMMAND.COM or lower level DOS functions) parsed those characters as special devices.
I think you could make those files manually in a disk or sector editor just fine, and it might even show up in DIR, but of course accessing them via normal DOS usage would be hard if not impossible.