I dont remember ever seeing this happen. Sure there are sometimes UA employees (either in or out of uniform) in FC, but they always board very late which implies that it was on a standby basis.

I have never been standing by for an upgrade, and ended up watching UA employees being called to the podium before me, and I have never missed out on an upgrade and seen UA employees in FC.

All this doesn't mean that it hasn't happened, but it is certainly not a common occurance as fly747first implies.