I've only kept track for the last two years... 3 times at check-in on short-hauls, once onboard on AMS-SFO.

Previously, I remember a number of times when all paying WBC pax would be moved upstairs, and elites moved to the main deck biz seats, with Y service. This mostly on 744 service from the Far East, on routes with more tour groups than premium traffic (ICN, KIX, NGO, among others). This practice seems to have disappeared with the shrunken WBC cabins