I'm not sure about the set up at YUL, but at other airports where there may not be someone at the Exec/SE/E check-in desk, I still stand in line and the first agent finished at the regular desks will wave me over. That said, AC has a pretty miserable record of properly staffing their Exec/Elite check-in desks at most times of the day. Another alternative would be to call the Concierge number on the SE card and s/he would take you to a desk for speedy check-in.