I've dealt with the NPS for a couple of Indian Wars events and to put it politely, it's a lot of BS. The only reason why I put up with it was the opportunity to do living history on some actual sites.
While we didn't have it so bad for Indian Wars, be prepared for all sorts of BS restrictions on weapons (not to mention DC's gun-unfriendly laws) and even on how you do your impression.
One event I did, we were issued a ten-page single-space set of rules and regulations which pretty much limited us to just standing still and saying nothing (OK, maybe that's a bit exaggerated

).
Anyway, I don't mean to paint a dark picture but you need weigh out if putting up with their BS is worth it. In my case it was since we got to use historic facilities.
If there's a coordinating organization acting as a buffer between the reenactors and the NPS, it might be a better situation. While I've run accross some OK individuals, there's the overall NPS mind-set which often is very reenactor-unfriendly.
Personally, I steer clear of having anything to do with the NPS but your mileage may vary...
