I agree with Vain - it takes time to prepare for a race, so would be frustrating not being able to race. It used to happen in F1 a few years back, when they had too many cars for the grid. Slightly different for a professional driver not racing though; they still get paid!
Hotlaps would be fair, but that can take up a lot of time, especially if you're struggling to make the grade.
Superpole would be good. Takes a little less time to prepare for, and the pressure ensures that the outcome will not always be the same.
If (or should I say when
) more people join, perhaps a 'b' series could be an option?
Whichever is adopted, I think that there should be a penalty for anybody that doesn't turn up for a race without advising the organiser. There's enough interest here for a full grid, so it would be a shame if people are excluded and the grid is not full. Could be something like a one race ban?