I typically find that just in case something like that happens (Y fare or SE) they actually don't let all the seats get assigned at checkin. They will leave a few (very few) seats un-selected in an overbooking situation and slide their "See Gate Agent" SE's and Y fare's into a seat before calling for volunteers - I've seen it happen very often (as I try to volunteer as often as I can!)

Speaking of which, looks like YYZ - YVR flights tomorrow evening are pretty packed, here is hoping for another voluntary non-boarding (to make it 4) but we will see how it goes tomorrow.