When I key the radio while the teller is NOT talking, I can't be heard at the drive up window. I've verified this with a friend.
I have tried your suggestion with every PL tone and DCS but have not verified them all. I really doubt they would use DCS either, seems highly unlikely.
One thing I've considered is pulling into the drive thru after hours with hopes that they leave the drive thru radio turned on. I would be able to tell as soon as I pulled up, when you pull up to the ordering area, a sensor(?) detects the presence of the car and goes active. I would then be able to try all PL tones while sitting there.
I have a Yaesu FT-8900R.