I think that this is where the IHSAA police come into play. That's there job to sort that out.
My opinion is that if a club coach is running open mats at the Y, and that club coach has no affiliation with the high school program (meaning he doesn't attend their practices, comes to meets and pays to get in and sits in the stands, etc) and kids aren't being forced to go then it is fine and it's not skirting the rules, finding loopholes, or anything like that.