It sounds like the OP has abandoned the thread, which is both unfortunate and unsurprising. It's so rare that real issues get discussed here.

The field where I fly is much like the OP's in some ways. It's in a county park reserve, and the county does all of the maintenance for us. In fact, it sounds like his field is exactly as mine used to be.

The field I fly at was created as a joint venture between a neighboring club and the county. The club was in the process of changing locations, and was apparently without a field. This was perhaps 15 years ago. They approached the county, and were able to get a field set up in a somewhat popular park reserve. By the time I started flying there in 2000, the club had their own field again and had pretty much vacated the county field, with the exception of one event every summer.

We don't require AMA where I fly. We have no chartered club. The county has two requirements for flying there. The first is that a person purchase a permit. I think we're at $40 this year. We'd been at $15-$20 for many years, but we encouraged the county to up the price. We've all felt like we were getting too good of a deal. The second requirement is that a person either have an AMA membership or that they have a rider on their homeowner's insurance that specifically deals with RC flying.

There are perhaps 15 of us that are "regulars" there, and I'm pretty sure we're all AMA members. We've never had to form a club, as the county is self-insured. We've offered to form a club so that the county can take advantage of the AMA's insurance program, but the county's attorneys have never felt that it was necessary.

I have no problem with flying with anyone at my field. So long as they have a permit, I'm satisfied that they're covered in some way, shape, or form. The county has always been good about checking this. Not having to have a club certainly eases our workload. The county maintaining our two runways, pit area, parking lot, picnic area, and toilet sure is nice, too. We're quite spoiled.

Unfortunately for us, progress and the popularity of the park are endangering our field. The county has had plans for several years to relocated the field to some undesignated location. Thankfully the parks budget hasn't allowed them to make the changes that would require our relocation yet. We've also worked hard at forming a positive relationship with the maintenance supervisor, the crew, and the parks planning manager. Our site is really on a year by year basis, but they've been willing to work with us. Not as much as some of my fellow flyers would like at times, but the county has gone so far as to re-route a planned bike path so as not to require our field to close.

Taking the time to form relationships with the people involved has really benefitted us. We aren't a strong lobbying group in terms of numbers. We'll never have the clout that the horse riders, dog walkers, and hikers have. What we do have is an almost complete lack of problems for the parks department and a unique attraction within their park.

Whether or not a site is mandatory AMA or not is really up to the land owners. All one can do is lay out the risks, make them aware of the services the AMA offers, and let them make informed choices. There's nothing to say that a club can't co-exist on public land with non-club flyers. It's just a matter of communication and courtesy.