For the CAN to US portion, I have flown from YUL and YYZ 20+ times in the past 2 years and they have been able to put my standby on a UA flight or even an AC flight. Several times the AC Flight would be leaving before UA and they just unloaded me from the UA and AC put me on their flight. Also holds true for UA flights and going on earlier UA flights.

It may be the policy online but I have never been not allowed to do complimentary standby in either CAN airport I travel through.