The $21 is the towncar rental from Disney. As far as I know, that is just for a transfer on property. We rented one to take us from the Boardwalk (our reception location) to take Grand Floridian (where we are staying the night of the wedding). If I had to guess, I would say no to the off property question. I dont think the drivers can.
My suggestion would be to use an off property service. For our wedding party transportation, we are using Lorden Transportation. Great prices and Dennis the owner, is great! They have a website.
Hope this helps!!!
Jamie