I would think that AC would be the only airline on which this is remotely possible. Even then, it seems like a stretch...they'd almost certainly want to connect you in YUL or YYZ, not YVR. You'd need BRU-YUL-YVR-GRU to be an allowable routing, and you'd need AC to actually sell these kinds of tickets. I don't know either.

Another option would be to find a BRU-GRU R/T that (a) allows a stopover and (b) connects somewhere in the western part of the U.S. Then you snag a cheap R/T to YVR or BLI (or even Seattle, if necessary) as a separate ticket.

This assumes that you can enter the U.S. and that the simple triangle trip really is 3x the cost. At some point, the time spent outweighs the cost savings...