Umm guys, actually ALL the Multi-Williams, Mega-Williams, and otherwise combo-Williams games are unlicensed bootlegs. Unless something has changed in the last week or two. The Mega-Williams "boardset" is a PC motherboard sitting on a JAMMA fingerboard that boots from a ROM, and apparently runs MAME, fact is, if you put a bigger flask disk (or whatever it is using for storage), then it could run 1000 games.
Multi-Williams is an add on to an original Williams boardset that lets it run all the games from that hardware.
As far as I know the only legal new Williams games you can buy are on the Ultracade.
Anyway, if the Mame devs or the Mame community actually cared one red cent about legalities then bootleg versions of games would not be in Mame.
Anyway, I am assuming you were actually wanting to emulate the Multi-Williams menu. If you had the ROM image of it then it should just about be a 30 minute project to add it to Mame (for a coder), since it just runs on basically unmodified Robotron hardware.
Guess what, all the new 3 in one and 6 in one Jamma combo boards are all Mame as well (except that they don't seem to be x86 based). The major supplier of them was on the newsgroup a while back talking with people about what games to add, none are licensed, and the boards will technically run ANYTHING.