First there is not a 100% chance that the same plane will be used on the YYZ-LGA as the LGA-MIA flight. I have been on many flights where the 'thru' (same flight number) plane as changed (and accompanying gate change) at the stop. Just yesterday in fact MSP-ORD-SFO required a different plane and trek to a new gate in ORD for thru pax.

NOW assuming it is same plane .. you may be able to stay onboard .. they say this at times on the PA.

If you get off, you have to take all your stuff and re-board (with the YYZ-MIA boarding pass) when other boarding is called.

Your last statement 'is this a viable option .. overnight stay' confuses me. What you are asking is fly YYZ-LGA . get off the plane .. hope for oversold flight .. volunteer for bump and spend the night in NY???

Kinda doubtful in my opinion.

The ticket you are on is a one segment (thru flight). The fact you get off and want to be bumped makes it a two segment ticket .. and that will make it harder for the GA to manage as compared to someone going LGA-MIA only. I am not certain if the issue is fares or segments .. but would think a GA would avoid using you.

Is this what you are looking for??

To me, the advantage (pun not intended) of a thru flight is being able to stay on the plane. Now as PLT your upgrade (if cleared) would be on the total trip YYZ-MIA and the mileage accordingly used for stickers. If you want to upgrade ONLY the LGA-MIA portion .. I am not certain that can happen as you are on a thru fare/ticket and to the computer it is one flight that just happens to have a stop.