The foundation for a national competition of any sort doesn't start at the national level, it starts at the root. Would be fairly easy (it would seem) to make an easily trackable competition schedule and gain entrants at every public launch nationwide. Anyone from TRA or NAR or perhaps even "backyard launches" that can be vetted somehow could form the bulk.
Entry fee per launch for a "recordable flight", $1/$5? Put it in a pot, either with the club/launchsite or in the pot for the overreaching comp. Money can be used to send each local winner to a state or national comp, or it can be added to a prize pool, or both.
Perhaps even just donated to a rocket related cause.
Or better, getting this sponsored from a true grass-roots level starting at local high schools or grade schools to get more people interested.
It's really not that hard to post validation for this kind of thing. ... Maybe I should start a thread based on this idea of just making a new LPR/MPR competition and see where it goes. hehe