For what it's worth, I flew YXU-YYZ-YQB last night, with the last YYZ-YQB flight of the day arriving at midnight.
YXU-YYZ was two hours late, with a few minutes to spare to catch my connection to YYZ-YQB so I've decided to drive to YYZ instead (1h20min).
I had my boarding passes already printed, told YXU check-in about my plans. She said no problem, I'll remove you from the YXU-YYZ segment and your boarding pass for the remaining segment is still good.

So technically that is not a no show, that's why it worked in this case.

(and the YXU-YYZ flight did arrive at the gate 10 min before my flight to YQB but I have no regret as I have spent enough nights in YYZ already (and I got an hour to spool down in front of a Glenfiddich in the MLL))