Ah! So follow up on my old post (ha) I have been able to find a pool that let me wear my mono fin. I emailed ahead of time but got no reply, so just went in with it. I didnt use the skin (its a finfun) but didn't hide the monofin from view, just didn't mention it. Said hi to the life guards, got in, did a couple of normal laps then put the fin on - no one said a word.
I did swim in the adult-only evening portion of the pool times so perhaps that helps? I can see why they wouldnt want things like that when it's busy or around kids, in case it encourages them to bring in things that are similar which perhaps are not as safe.