I'm sure there are a couple of frustrated 1K on UA 1556 today . . . my GF bought a last minute ticket this morning, showed at #16 on the wl as a Silver. 2 F seats left, and - remarkably - PZ1. I applied PP, and voila, confirmed in F. The other F seat disappeared soon thereafter (no more upgrades shown) . F now booked full.
This is why I pretty much assume that if I don't clear at T-24, it's either buy-up or sit in back.