Originally Posted by
cfischer
I am very certain of it

And yes, it was on UA ... and yes, recently as well. Where are you reading this is not possible? It is automatic when on the same reservation, but I have most definitely had GAs put a companion into F when they had unsold seats prior to departure.
I am not aware that it is even possible to upgrade a companion who is not on the same PNR. I have always applied my RPU when my family travels with me and we're on different PNR.