The Fan Game - Back to the Future Part III: Timeline of Monkey Island they call it, and it looks right about as wacky as it sounds.
From what I understand, you follow Marty back to the times of Monkey Island in an alternate timeline from the original games. I mean, of course it’s alternate; that’s the effect of time travel. Anything else would be crazy.
A demo and all is apparently available: More on Twitter
(Thanks to Benzo for the original heads up on this.)