So you ignored the legal advice bit. Can we agree then that there is a legit dispute and that if ever there was a time to have a chat with an IP lawer about how to go about re-licensing files in MAME this would be the time to do so?
Why the assumption? Because it'd take an incredible amount of time and persistence to do with absolute certainty and without error. Further, I suspect there's has to be some very close calls in there ... who is going to make those? Once done, there's the task of contacting every contributor and not moving forward until you've received a response or re-written their stuff. That seems almost insurmountable unless there's some really sweet tracking of contributions going way way back.
But hey, if people are putting in those thousands of man hours then kudos to them ... should have said-so right off the bat. However, if a contributor doesn't feel like combing through every file in the project to see where their code has ended up I can hardly fault them.