Just because something is paid for on your behalf doesn't mean you shouldn't tip.
Example: If a friend of mine is the executive chef at a restaurant, and he buys me a round of drinks, I should still tip the waiter the normal tip for these drinks as the waiter still went from Point A - B with them and would have earned that money if I were paying.
The Boston Coach driver picking you up would normally be carrying someone that called for a car, so he would be tipped.
I would play it safe and tip. Afterall, you are getting a free limo ride, so you are ahead of the game anyway.