"It was working when we took it out of the cabinet, haven't had it hooked up since so it should be fine, although I have no Arcade boards so no way to test it."
I just worry about this kind of warnings, it just tells me NOT WORKING.
Anyway, if any Arcade Monitor is 15khz, then AVGA will work. MAME states beastorizer as a 640x480 res, and that tells me its a VGA res. If it is a VGA then AVGA will not work.
Maybe there is something on the net or Ken Layton can exactly tell you what it is. Wait for some post...