Guys - they are, in fact, legal provided they have the required safety gear. This was verified by our campground owner last month when I asked. I would NOT attempt a trip to Gatlinburg, however.
As far as getting around PF, they are starting to show up here and there. Traffic generally runs slow enough that you aren't going to get trampled, however, I'd keep a CLOSE eye on my rear view mirror. After all, it is a town full of tourists who don't know where they are going.
A lot of the parking lots at the tourist and shopping areas are connected, so you could, in theory, not have to be on the parkway very much.
A few of the shopping areas a cart would be VERY handy, as parking is at a premium during peak season. But off season, there is plenty of room to park a dually or whatever TV you use.