Martinus
Emperor
Prague also pretty much rules them out, it was an absolutely vital part of the Holy Roman Empire and the capital for long periods of time. Another key city, Vienna, is also the capital of Austria. So yeah, whilst it's not impossible, the lack of Vienna and Prague, as well as the alphabetical achievement list (which could be wrong for whatever reason) pretty much rules them out.
Yeah. Besides, with no Prague, as well as Austria, France and Germany all in the game, there isn't really anything they could based HRE on.