I believe that there is a machine which will accept cash and issue you with a card if u have cash, don't know about Maestro, but if it was me I would come, ask someone in the queue to pay for your visa preferably from the same plane, if all else fails, give them the cash! I would not refuse and neither would many others! Speak up and people will help you, no one in Doha wants to refuse entry just because u haven't got a CC!