No bites yet? Guess I'll have to do some experimenting!
CLG, I've never done what you're attempting to do and quite frankly am in awe of the task you put before you.
I can say, however, that time is not your friend
when it comes to any kind of pizza.
Any delivered pizza is never superior to it's in-restaurant counterpart.
The closer the pizzas are to the time you pulled them out of the oven, the better.
If you have any kind of insulated or heated box for the pizzas, they can possibly get soggy from the steam,
and even if you have something to control that humidity, the pizzas will eventually start to dry out, the longer you wait to serve them.
20 minutes for delivery probably won't be too bad.
Preheating pizza stones and using them in pizza bags sounds like you're adding a ton of weight to those bags without a really big payoff, but it could help.
I imagine that the pizza industry uses a variation of those chemical hand warmer packs that people use in the winter to keep their delivery bags warm.
REACTRO Heat Packs:http://www.roshgo.com/Merchant2/merchant.mvc?page=rc01/CTGY/00604
If the normal insulation in your delivery bags/boxes isn't enough already, that's probably your best bet.
Worst case, you could keep that on-site oven on full-blast when you get there and you can re-heat the pizzas if they get cold before they get there.
When I've gotten Lou Malnati's pizzas to go, they put a sheet of plastic/wax paper over the pizza,
then cover it with foil and put the whole pie in a cardboard box, with the pizza uncut, all of which should help keep the heat in.
Good luck. Let us know how it goes!