> Absolutely. Is there a central registry of every contributor ever? > > Remember, part of the argument is that previous license changes didn't involve any > such super rigorous effort (correct me if I'm wrong) so you'd have to go back to the > original release. That is, unless every contributor was contacted in a legally > binding way, you have to keep going back to the original release.
Reasonable attempts at contact, with a clause in the license setting forth a grace period where authors can contact MAMEDev for code removal should suffice.
One could also argue that public notices in a few related and widely read trade distribution would suffice, with no further effort necessary. Its all about how defensible you want your stance to be, and how much money you want to spend defending it if the need arises.
(Personally I'd call discussions on Mamedev.org, mameworld.info, and arcadecontrols.com legal notice enough. I would hope/expect that the majority of MAME contributors went into this expecting nothing in return.)