I'd start by trying to get a group of like minded people together and start up a group that can take on these things. A "group" that can get organized and approach a park with a plan for trails- an actual map of the proposed trails system, a timeline for putting them in, the rating of each trail, the IMBA sheet showing what the ratings are, and a plan for maintaining the trails long term.

Have the person who may end up being the "trail Steward" for the park attend IMBA's trail building class when they come around. Then have the Steward appoint 3-10 people as the Dirt Crew. The dirt crew will help the steward get things set up so when 25 people show nup for trail day they've got somethign to do, AND it gives a 1st teir list of people the steward can call when someone tells him there's a tree down, etc...

You should make the "Trail Steward" the 1 person who deals with the park. The ranger doesn't want 10 people calling him up and trying to get him to be on their side when stoopid arguments come up, and they do- all the time. You'll be shut out in a heartbeat if you do this. Since the trail steward deals directly with the ranger and other park reps, the steward knows what is expected of the club and the desires of the ranger. Therfore, within the park the trail steward is GOD, and no other position or person within the group can over rule him or her. Not even the president.

It can be TOUGH to get started- remember that park rangers are approached almost daily by people wanting to do this or that on their land. MOST want the rangers to finance and build their projects for them. If you can offer the amenities and show you have the ability to pull it off you may have a chance, but you really have to have your poop in a group to get a large park to give you a chance.

FORC had the good fortune to find a chunk of land that a city owned and wasn't using for anything. Some of the old school doods got permission to build some trails there- and as the popularity grew, so did the parks amenities and now that chunik of woods is Sunderbruch park, in Davenport Iowa.

Contact Hansi- he's good. Keep asking questions, and good luck!!