I wondered about this myself just before Christmas on a WT+ trip to GRU (especially as it was the first time out of J/F for a while!). In the end I decided that it was more hassle to worry about than it was worth... so I OLIC'd 23hrs59m before as usual, picked a WT+ bulkhead and confirmed.

Met some friends in the bar at Paddington before the flight, arrived at LHR pretty late (about 50mins before) a little worse for wear, pondered about going to a checkin desk in the hope of a SFU comment but again decided not worth the hassle.

Put my card in the SS machine and out came a J boarding pass... I was so pleased! (I remember in my slightly pissed state thanking the SS machine profusely and getting a really weird look from a couple of people nearby

The conclusion, at least for me, was that if its going to happen (status, overbooking, OLCI, desk, no desk, whatever...) it'll happen.