A photocard driving licence should be fine as long as it has any number on it that they can input as a document #. IME, once they know they're dealing with a foreigner, most people aren't really that fussed (espcially when you're spending a lot of $$ - turn down the sale or bend the rules. Tough call!!)

Another option which worked for me without fail before I got my Argentine ID was to carry a color photocopy of the photo page of my passport shrunk down to credit card size and laminated. It was never refused (it looked just like a laminated ID card) and saves the worry of carrying original documents around with you.