Don't know if any of you guys have ever been up there, so I don't know if you know what the roads are like first hand or not. But I grew up and lived in central maine up until a few years ago(millinocket area), and the majority of woods roads around there are quite nice, as you said you could take a car on some of them. To get any decent off-roading, from what I am used to, you need to find old skidder twitch trails that go off of the dirt roads.
Sometimes these twitch trails can be quite steep, and quite rocky as well, some times with mud on the way as well.

But you are right, a lot of the scenery would be great, especially if you went in the fall around middle of september to middle of october. I have never been wheelin' in far northern maine, so I can't say for sure what it is like. But there are hundreds of roads up there though, and some incredible fishing. Hope this has been of some help.