LE, tell the agent to short check your baggage to YYZ and that you will terminate your travel there (therby also taking you off the PAX list on YYZ-YUL).

I have done this before at YVR and had absolutely no problems.

If the agent gives you trouble, just tell him / her that you're meeting a friend at YYZ and will require your luggage as you have something for your friend and it's inside the bag. (I've done that at YOW.)