WorkingHard: Sorry to hear that your offer to participate hasn't been taken up in the past - more so that no one got back to you to let you know why.
I'm not involved with the rostering, but I can tell you that we are generally over-subscribed on two counts:
1) from the 'usual' favorite locations, where we have more seats than bums
2) in landing slots. At Kemble in 2002, we had a physical limit of 125 aircraft - it would be temporally impossible to land any more on a single runway by the start time, refuel them, and get them all off the ground again at the end!
Unfortunately, there are often a few aircrew out in the sticks that end up getting left behind because we haven't got and aircraft near by, whilst there are spare seats available at Waltham, Barton, Elstree, Popham etc.
We always accepts as many registrations as we can, and then roster to get the most aircrew there on the day given the number of aircraft/seats available. Dennis does take care to see that as few as possible are let down, trying not to leave out the regular attenders nor the ones that have never been before!