It can't be mutual because the receivers don't broadcast, so the sender doesn't know which contacts are in range.

I was also thinking you might be able to use asymmetric crypto for this, and encrypt the hash + a nonce using your private key, and anyone with your public key can decrypt it and check the hash against the contact list. But this means the potential receiver needs to decrypt with every public key it knows, which for large contact lists might be prohibitively expensive.

Someone has probably devised a more clever way, though.