Brad my feelings are that more or rather different questions need to be asked here, to get where you are going or thinking.

Those that are looking to do some off-road racing on Wednesday nights.... that is the question posed, is that in add to their Friday or Saturday racing? If they are only going to move from racing on Friday or Saturday to the Wednesday night racing then that will dilute the current turn out with no net gain.

I think one way to counter act this would be to have a series. This series would be taking your best main each week and run it for 6 weeks... compiling points from their best single weekly results. This would allow racers to run an additional day during the week, to post their best time. If they run earlier in the week (Wed) and they think they got the best run for the week, they may not race the rest of the week. However, If their results were not good in the Wednesday main then they can come back Fri and/or Sat to try and improve their week's result. You could pose the strategy for racing Wed nights would be if turnout is low then that could equal a clean run but less traction. Where as Friday and/or Sat would be more traffic with max traction.

Another idea, try to come up with better ideas to reward middle of the pack guys to keep them interested in the series. Rewards for most improved over 6 weeks or most consistent under 26 laps, something along those lines.