Took me a while, but I finally think I understand how relays with Access Control Lists (or paid relays) can do their thing.  It's not a gossip related thing, it's simply a client thing.  If clients do their normal gossip thing, when they encounter an inbox that is paid, they can simply show this information to the user.  Nostrudel has a nice way of popping up this "toast" notification for everytime you publish a note, it shows the relays that succeeded and failed.  So if clients just show this, they can say "hey, this note didn't make it to these inboxes because of reason: X"  Retry? Visit Relay landing page?

I find myself watching the gossip logfile all the time to see this kind of thing, maybe gossip client can show this user feedback as well in the future, and then there doesn't need to be a disclaimer about using inboxes that have Access control or etc.  (Or for upcoming when more relays rollout READ protection via. auth, etc.)