Really, the availability listing is about the best you can do. Sometimes, a reservations agent will give you a little more information, such as how many seats oversold the flight is, but a) I don't think they're supposed to give out that kind of information in general, and b) the number of seats oversold doesn't mean that much anyways unless you know what the typical no-show rate is for that particular flight on that day of the week, etc.

If you take the same flight every Monday, for example, knowing the # of oversold seats may be useful, but in general, it's probably not going to tell you much.