Hmm...fun to consider!
Besides your suggestions, nothing jumps out at me... It's not so much that Peter and Lois would would make a good George and Lorraine as it is that Lois jumps out as being a surefire Lorraine. Mainly, teenage/1950's Lorraine. Since Peter is Lois' husband, him being George then seems a given. Although, adult Stewie would make a pretty good George, but if Lois were to be Lorraine, that'd be kind of weird...
As Stewie has the whole "mad inventor" thing going for him, he screams "Doc"...but, like you said, he's Peter and Lois' son, so it'd make sense for him to be Marty. And I think Stewie could pull off Marty's scrapiness. I can almost see Chris as Marty, though he already had his turn at being the lead in the Star Wars parodies.
If we rule out Stewie from being Doc, I really can't come up with anything else. Brian at least likes to pose as being an intellectual, and feigns that he's serious about science insofar as that he thinks it validates his atheism, but he just doesn't quite fit as Doc? Adam West? He may be a screwball...but he doesn't really fit. Mort? He may be bug-eyed and neurotic...but he doesn't really fit (too much of a wimp, while Doc has nerve). And West and Mort are both too distant "from the main cast". I'm really coming up with nothing here. (Yes, I realize that every FG regular would be an "actor" in this scenario, but, like with the Star Wars parodies, I'd expect that they'd be a match in some way.)
I'm seeing Joe as Biff, and Quagmire as one of his lackeys. But do you think it would be okay to cast Joe as the main villain?
I'm thinking Herbert as elderly Biff...but having different "actors" play the same BttF characters at different ages seems like it would feel "off".
Hmm...could Herbert be Biff? ...lol, Herbert as Doc and Chris as Marty! ...woah, that might actually really work!