You cannot SDC to a routing not permitted by the fare rules, period. For YYT-YYZ-YEG, you can SDC to YYT-YUL-YYZ-YYC-YEG, but whoever does it for you will need to issue a FIM for the two additional segments, and it's a process an average agent will be unwilling to go through.

Under IRROPS situations when you've been delayed several hours, you can request additional segments if they will somehow get you to your destination sooner, and your argument would be more convincing since the change is involuntary. Since a SDC is a voluntary change, the fare rules stay the same as the original booked fare.