When dealing with UA for a US-SCL ticket, be aware that from many cities the xyz-YYZ-SCL Air Canada routing exceeds the xyz-SCL MPM (Maximum Permitted Mileage) and thus UA won't allow it.

UA *A awards have to either be within the city-pair MPM, or they have to follow a published routing. YYZ is not on any of UA's published routings from my home airport.

You may be able to talk them into an exception by accepting longer routings that have most of the travel on UA (and thus less cost to UA than if they actually had to pay another carrier). For example, they gave me DEN-IAD-EZE (UA) connecting on the AC stub from EZE-SCL down, and SCL-GRU (LX) connecting to UA for GRU-IAD-DEN. Much further than the MPM, but because the intercontinental legs and the travel to the intercontinental gateway were all UA in the states, I guess it was ok.

But originally they were telling me there was NO way they could get me an award ticket between DEN and SCL. It took much insisting that SCL was a valid Star Alliance destination in South America and that UA published a *A award schedule between North American and South America and thus had to at least look for availability, to get them to try.

With you in LAX, I think the likelihood of them allowing the YYZ AC flight will be even less than in my case. Some NYC-based FTers have reported success with getting UA to do it for them, if I recall correctly. Perhaps either within MPM or at least less of an indirect/backtrack routing.