Earth Tuner,
Here is what I found. I just got back into the hobby again. I Google'd my area and found the same thing. So, here in Lane County, OR we have a park district called Willamalane. They own and manage many park sites. One very near my home. I sent an email with my intent to use my detector on their property but let them know I wanted to ask permission before hand. I explained I use the techniques that preserve the ground as best as possible. They were very happy for my checking in first. Sent me a permit in an email, I just have to stop by their main office to get it validated. Then if I am approached by others that are not in favor of our hobby, I have a written permit from the park management to show them. I have found that if you are genuine, ask first, and see other's perspectives, you normally get what you are after. I would shoot some emails to the folks that manage the areas you are interested in. I would be shocked if you get a NO in reply.