I went through the rulebook and could find nothing at all relating to capacitors. I would assume from that that using capacitors in place of batteries (if that's possible) would effectively disqualify the car from competition. There is no designation in the "batteries" section of the rulebook for the use of capacitors in lieu of batteries, so there would be no class for a car so equipped to run in.

As for running capacitors in series with the batteries, I could find no rule against it. I have to wonder if it's been tried and what the results were...