I have not used plateanet, but I have bought tickets on Passline this week, just using my US passport number. The idea of travelling 45 minutes across town, during the exact hours the box office is open, then back again, to save 500 pesos must mean you have a LOT of time on your hands. (I am referring to shows that will sell out before the actual event- which most of the events I have been to or plan to go to are doing).
Most online sites beside Mercado Libre will accept a foreign passport number instead of a DNI. Passline actually uses Mercado Pago as its front end, and it worked fine with a passport. Add a couple of zeros at the end to make it 11 digits - that has worked for me sometimes in the past.