You can buy MLL access for $25 on AC (when in PY, it’s more from Flex Y). There’s a Intl MLL in YUL, they’re not the best lounges generally, and I’ve only been in the Dom one there but there’s a thread about it on the AC forum if that helps.

FWIW, I always buy MLL Intl lounge access when flying AC PY. At least if I’m going to be there a few hours. However you do need to buy in advance and if you get delayed and don’t have much time I doubt they’ll refund it.

Not sure on other lounge options at YUL. Others may well do though