We've tried this set up with a professional photograher at other events I've been involved with.
You'd be surprised how hesitant folks are to step up and take advantage of this service.
I don't know if it's the price or what. You hear a lot of "we'll have to come back and do that" but you guessed it, they never come back.
This set up would have to be somewhere folks could stop at while heading in or out of the rally site, making it as convient as possible!
I think a lot of folks also want the digital file for use in other formats and most professional photographers aren't going to part with the file.
my 2 cents,