Well, I don't really know much about williams boards or cabinets, but I have heard of Williams to Jamma to adapters. So mayber 2 adapters plus 1 multi jamma selector would work?
Or you could just build the selector yourself. I'm sure some of the guys on this board could give you the pinouts.
About the button: I would not drill a new hole. I would put away the original control panel and make a new one that has the extra button. That way you can have clean conscience and play the games the way you want to.