Once last year I had asked for an upgrade using miles on a flight where the only seats available were revenue. (My wife was already in first on a reward and I didn't want to hear how I "left her alone"). They promised me there was a "waiting list", and that as a Gold (last year's top) elite, they would contact me once a seat cleared.
As it turned out one never did, and when there were only 3 seats left at all, I paid the difference in fare to get the upgrade from an H fare (which is not restricted to F).
I don't know if they would have called me, but I certainly called in once a week to check on it.