Is this regarding that Ms Pac you posted on KLOV? I didn't read the thread, so I don't know what you have planned, but if you are not planning on butchering the machine in the process, I doubt you would get much hate around here for putting in a multicade.
Yeah, you'd get suggestions to restore it to original, but we get more upset when someone takes internals and junks them, tosses away hard to find specialty controls, junks working CRTs for a widescreen LCD, chops notches out of the sides into to fit a giant frankenpanel, or defaces original art for some corny montage of video game mascots.
The big question is, are you doing anything that would make restoring the cabinet back to original (if you so chose to do at a later date) much more difficult? If not, you are probably fine around here. You can generally toss whatever you want into a cabinet without ruining the original hardware.