So after digging around, I managed to find a little known Mega Man compilation called Revenge Master and, lo and behold, it just so happened to have a crapton of rare or modified Robot Masters mostly in the Power Battle style that, as far as I can tell, can't be found elsewhere. But rather than just leaving them in that compilation, I decided to reupload them to the net and release them to the general public so that everyone can get them and use them in their own MUGEN's if they so choose, and they're all available in one big pack here: http://www.mediafire.com/file/754ut95525hvmji/Rare+Robot+Masters.rar
Also, here's the compilation by Fakeman in case you're interested in that: http://www.4shared.com/rar/-plhnRoRce/__rar_02.html?
As for the Robot Masters in question, they're as follows:
Hard Man by Calamix
Wave Man by Calamix
Burstman by Magma Dragoon MKII & O Ilusionista
Bomb Man by O Illustiona (Done in a 2D Powerup style, so that should probably go in the Classic thread,)
Charge Man by KhMaIBQ
Drill Man by KhMalBQ
FlameMan by ShoShingo
HornetMan by KhMalBQ
Knight Man by Jher
Metal Man by Laspacho
Concrete Man by MegaMan X4
Fake Man by MegaMan X4
Snakeman By Heyakes
WindMan by KhMalBQ
YamatoMan by KhMalBQ
Crystal Man by Roberto22(dragons)
Additionally, Burstman, Knight Man, and Metal Man are all based off of their respective MvC-styled versions. Their sprites and moves are taken directly from them, but their gameplay's been modified and their sprites resized by Fakeman so that they play just like the other PB characters and fit right in with them. No normals; just specials, most of them done with one button press. As for Hard Man, Concrete Man, and Fakeman, they all have their NES sprites, but resized and recolored, and with PB gameplay to boot. They're somewhere right in the middle between PB and Classic, so I guess they could go on either board. (Except for maybe Concrete Man, since he wasn't actually recolored; just resized. But whoever edits the opening post can be the judge.) And that should it. If I find anything else, I'll be sure to post it right here.