Short story, I am Australian , currently living here with my Argentine partner, I had overstayed my visa due to our car breaking down on the freeway to catch the BQB boat at Easter (last day of visa last seat on boat!) so we booked tickets on the 30th July through a Buquebus agency for Saturday the 3rd August 8:45am to travel to Uruguay, we arrived to the Buquebus terminal very early allowing time for the trouble of the Visa (I was also overdue my Reciprocidad fee of $100USD) Buquebus "check in" held our tickets and told us to check with Migraciones upstairs, after much fuss, Migraciones were happy to let us go through but on taking us back down to BQB, they said we would need to go to Migraciones (near BQB) we ran there and they are of course are not open on Saturday......back to Buquebus and they said we would need to go to Migraciones (Terminal de omnibus en Retiro) and they would change our tickets for the later boat. After a broken down Taxi we were able to pay the fine of 300 pesos but could not pay the yearly Tasa De Reciprocidad (Yearly fee $100USD for USA Canada Australian). We changed our tickets for the Sunday (4th) so that Saturday afternoon we went online and paid the Reciprocidad fee and printed the receipt to take with us through Migraciones. Sunday morning we went through customs after 5 people trying to sort out the problem of the computer not reading a number on the paperwork............apparently this has just changed on the 1st of August and I think we were the first test dummies to sort their internal problems.
Note: Make sure you take your Reciprocidad receipt of payment with you for the return back through customs in Uruguay.
This is just our experience so maybe one of the administrators might wish to look up the ruling and post it?
Good luck!