One time, a couple of years ago, I flew to Beijing and my upgrade didn't clear. The concierge in the lounge was the one who told me that I hadn't made it ... and then she phoned to the gate, said she had an SE who's upgrade had not cleared, and asked that they block a row of three (Y) seats for me.

Obviously there had to be space available - as I recall, there were a decent number of economy seats open on that flight - but I didn't get the impression that there was anything within the system to formally block seats, it was more along the lines of "as you manually move people around, please don't put anyone else in these seats".