There is another alternative, if you don't mind clearing Canadian customs/immigration. Just ask them to short-check your bag to YYZ, saying that you're meeting a friend to give them a gift, etc.

Then go through normal immigration/security, grab your bag, and go down the connections hallway. Simply walk to the left before the doors to go talk to an agent (I don't bother waiting in line or talking to an agent, just tell them I'm rebooking and bypass them), and ask them to rebook you on an earlier flight.

Standby or not, they can then tag your bag, throw it on the conveyor, and you go up to the US entry doors.

Risks: YYZ takes 25-40 minutes for your bag to hit the belt. However, if you hit the connections desk you'll have a better chance of getting on an earlier flight, than waiting for the transfer, US immigration, and then asking the desk. You may also be able to play the "my flight is boarding!" and perhaps skip to the head of the line