I might be completely wrong here but as far as I was aware, tourists have to have their passports with them to book/travel on a long distance bus trip within Argentina (I did going from Baires to iguazu - Arg side)...but for the bus company, not for a police check. I don't know whether they'd check your visa or not if you weren't going to be crossing a border...or care if it were expired. Check with the bus company.
If I were you, I would avoid carrying a passport at all costs, especially if its not valid. I've never seen seen a policeman ask anyone for their passports randomly.