Not DCC but I had an interesting experience at a store in MEX post-security. While buying a new phone charger, the clerk scanned my boarding pass and passport and proceeded to ring me up in USD. (Prices on items were marked in both MXP and USD.) The person in front of me, on the other hand, looked to have a Peruvian passport and was given a currency choice before her card was inserted. I didn't see any prompts on the PIN pad itself, which of course was on the cashier side as is Mexican custom.

Anyway, I'll have to look at the receipts more closely to see if MXP would have been significantly better.