I don’t think the coach is off-limits. I think if YOUR kid had a problem with a piece of equipment, a medical device/intervention, etc. and asked if you had any suggestions- and you tinkered and found one- and it worked– THEN forwarding to the coach could be perfectly fine with your D’s permission.
It’s getting involved in another kid’s issue (boundary number one), assuming that the coach hasn’t already tried to get the kid to ask the physical therapist for help (boundary number two), volunteering that you had a solution and here it is without being asked (boundary number three) that starts to feel like a pile on.
Do you welcome unsolicited advice from people who aren’t employed by your company or who are not customers/shareholders, etc. telling you how to do your job better? You may be fine and grit your teeth; you may resent it but are perfectly polite, or you may think “Wow, what a jerk”. Or some other reaction. But few people are excited to be told by lay people “I can do it better”.