Yeah, realistically speaking the highest amount of Dukes fans they could pull for a JMU home game is ~30K. Being extremely liberal with an estimate by adding another 5K of noVA alumni, that would be 35K just from JMU (20-25K Dukes fans seems like a reasonable estimate). I don't know what kind of numbers WVU games bring, but I'm sure it would be close to double that.
The encouraging news is that I have yet to talk to a single alumni that is NOT going to the game, even those that aren't sports fans. So there should be some good purple/gold representation there, as well as some JMU style tailgates.