Sorry I haven't gotten back to you about this sooner.
I've thought about this as well for making a top down driving game. You can calculate the friction forces on the wheels easily enough and apply those to the body.
The problem is that the friction force won't quite have the stopping power that it should have unless it is made into a constraint that is solved along with the collisions and joints. This problem will really only show up in the case where you are pushing an object that should be held in place due to friction. Instead of staying put, it will slowly creep.
I've been thinking about doing away with joints in place of just generic constraints. (really just a name change) Constraints could include other things such as top down friction (for lack of a better term), damped springs done right, motors, etc.