Oddly enough, the smart does come with anchors for a baby seat. It would go in the passenger seat, of course... the rear is not a place for any form of passenger. The reason for the rule that you don't put a baby seat in front comes mostly from 1990s era air bags. The front air bag turns off if the passenger weight is under about 120lbs. Even so, in a 4 passenger car, the center of the rear seat has more of a "crush zone" around it and is usually safer for child seats only... an adult will fly forward where the two on the sides would be 'cushioned' by the seats in front of them.
It's still a 2 passenger car, though. Sorry, moviestar, your wife will just have to put on roller skates and lasso the smart.