Nah, I want to have a prediction how full a plane would be. THAT would be a service worth paying for..
The two are not mutually exclusive. The higher the general vacancy within a given plane the higher the likelihood of there being cheap upgrades available immediately before take-off. I generally plan my travel around less desirable times because of the increased likelihood of a $50 first class upgrade on Virgin. Out of my last 40 or so flights, I've upgraded on 36 of them.