It's not a park, no rangers or anything, just woods and trails and so on. I'm thinking I'm good to go, but reading into it is starting to make second guess.
You have to ignore the uninformed gibberish that pops up when ever this subject is discussed and follow the facts.
On federal land in MI you are hunting coins and jewelry, in state parks that are open to detecting many have areas that are off limits and the website provides clear shaded maps for the open areas. Grand Haven state park = wide open.
Ludington state park the entire beach is open south of the main beach house. Newaygo state park closed, etc. It's not that hard to hunt legally here.
I buy year passes to several county park systems around where I live and every one is open to detecting, you just inform a park person on the way in.
Links to park rules in MI.
http://www.fs.usda.gov/activity/hmnf/recreation/rocks-minerals
http://www.michigan.gov/dnr/0,4570,7-153-10365_10883-97922--,00.html