The only time I've had a drive-thru request, the PAX said "We want to go to Jack In The Box if that's OK. Are you guys allowed to do that?" They were very nice about it.
It was my one chance to say Oh you're right, we can't. Sorry!
Anyway, we did and the car smelled like that for 20 minutes after. Had to drive around with all the windows open to try and clear it out for the next PAX.