How about treat the Nationals as a championship for the proffesssionals to do all rounds and then 'local' racers enter their local rounds. This would boost the numbers at each round without all the travelling involved. This makes alot of sense to me., the local guys are racing on their local track all year and then the national circus comes to town and they can show the big boys how it's done!
It would be a similar situation to having local racers enter GP's as wildcards (Ben spies on the Suzuki at Laguna Seca Moto GP last year for example)