I agree, it would be very nice, but with the current method of generating the mirror graphics it's impossible - short of simply faking the car model in there. The reason is that we currently have only ONE image that is generated for the mirror, and the side mirrors do nothing else but show a different part of it.
The "correct" way would be to have three seperate viewpoints for each mirror. These could then also contain the car model, but the problem is that it's very performance hungry for such a little effect. It would be nice to have as an option for monster-systems, though.