Using the words "AEROPARQUE" and "overstay" in an advance search I just found this:
gvas12, on 01 December 2014 - 10:26 PM, said:
"You can't pay the overstay fine at any old crossing anymore... two years ago yes, not anymore. You can pay it in Retiro, Ezeiza, or AEROPARQUE but it must be within 10 days of your departure!!"
PS: I don't know the answer your second question, "If so, is it open 24hrs like at EZE?" but I imagine it would be open prior to any international departures. If that's not the case, you can no doubt prepay the fee there or in Retiro during the day, but, as gvas12 notes, no more than ten days prior to your departure.