These days, would AA even allow this on an AA-AA separate tickets trip? My understanding is no.

I agree that the best chance is to get CZ to check the bag through (more because I know more about AA’s policies than through any knowledge of CZ’s policies). Once it’s tagged through, you’re pretty well assured that AA will accept it.

If there’s a return LAS-LAX-CAN on separate tickets, I think it’s very unlikely that AA agents in LAS will check through.