I don't think Universal is gonna pull a Captain EO here, and just putting the BttF film inside a Simpsons shell would be really strange. Wasn't it some big anniversary for the ET movie not long ago? And I don't think they even had special merch for that and the ride is still in place. I really doubt they'd have a BttF pop-up event in the Simpsons ride. If they even acknowledge the special BttF date, I would guess it might be like a "limited time magic" level thing. Maybe a Special M&G by the train and the Delorean, or special merch, or a special feature before/after CineSpec.