I recall way back when I was first on the forums (this one I think?) there was a pic of a tractor seat (w/ seatbelts) that someone had mounted in the back of their smart.
As I recall, it stated so he could take his wife and 4 yr old granddaughter "just down the steet" for ice cream?
Personally, I thought it was a unique solution, but also dangerous?
I can't find the thread or pics now (of course) but I would strongly suggest NOT doing this.