I've never had them stamp my boarding pass when entering any of the YVR MLLs on a MLL cert. As bakersdozen says, make sure to ask the dragon as you check into the lounge.
Originally Posted by
bakersdozen
If I recall your boarding pass gets a red MLL stamp when you check-in into the lounge using a certificate. This red stamp allows you to come and go as you please. I ASSUME since they'd stamp your pass in the evening you could use the same stamped pass to regain entry the next morning when they open.
It will probably work as you just have a long layover... make sure you talk to the front desk when checking into the lounge and just ask them.