So - we don't have the AS issue because Nostr is our AS. We use pubkeys verify authenticity and you're trusting the key package events signed by those keys when you're adding someone to a group.
AFAICT, the participation privacy question is about relays being able to see what groups you're in by seeing what group IDs you're requesting. I believe that we've mitigated this pretty well since we're using random (and rotating) identifier(s) for each group (yes, it can be more than one).
We also want to eventually break up the requests into lots of different reqs/subscriptions (probably done over Tor or something similar) to further obfuscate this info.