I don't think going straight to the top, is always the best way to get permission. You might get lucky, and get someone who is into the hobby, but that's really low probability. Each site, is an individual case, unless the whole town is off limits, do to a group of pirates armed with garden spades in the past. You really need to get the blessing from the whoever is personally responsible for a particular site. Don't think it's reasonable to get blanket permission for the entire town. Some parks are meant as recreational, so are meant to be quite, peaceful, heavily landscaped, meticulously maintained. A few parks, are like gardens, populated with unique vegetation to the area, which they don't want disturbed or destroyed.
So, officially, the town isn't open to metal detecting, but still, you might try the individual areas of interest, and someone could still allow it, take responsibility. Get it in writing, contact information for the individual.
It always helps to dig the trash, pickup a little surface trash, as you hunt. Try to keep you activities on the positive side, when hunting in public places. It will help, if there is ever an issue, and could open things up a little over time. I don't empty the junk out of my bucket ever hunt, just when it's getting heavy or full. Mostly, I hunt the curb strips around town. Lots of bottle caps and pulltabs, but enough 'treasure', and interesting items to make it fun and enjoyable for me. I've never asked for permission, never been challenged about it either. I stick to the sections, which are simply mowed occasionally, pass over the section well maintained (sodded, seeded, landscaped). Also stay away from the sections with utility boxes. I try not to generate interest or concerns, and most left to enjoy the hobby. I do get bother with questions about what I've found. Sections where there is parking on the street, are fairly productive, lots of keys and coins. Bus stops are pretty good as well.
