I'd just do OLCI. If it is too early before you leave YYZ, try @ MEX. You then go through transit security and then back up to departures. If you're flying up front, or have ST status, head to the Aerolineas Argentinas lounge (upstairs, near Gate 14 or so) for assistance if you need. I'd also head to the gate at the earliest opportunity for doc check with an AM agent. (Also, if it doesn't work before you leave YYZ, perhaps because an AM agent hasn't done a doc check, it may work after your YYZ check in).

Otherwise, you'll have to go through arrival formalities, and then departure formalities, and sometimes those lines are not pleasant, on both ends.