I already had a TAC case open with GM, so they shouldn't have to do anything. But then again, I never trust dealerships to do anything right/sensible.I have no Volt but I expect that it can run for minutes, depending on the heat load (temperature difference).
Did you check the Volt owner manual? I believe that you can run the Volt's gas engine while parked for testing by opening the hood. Then if the backfire happens the dealer can catch it while under test.