[talks about posting anonymous messages that only recipient can decrypt]
like a 4-bit checksum of the PGP key or the key length as a label - it's not enough to identify which key it is, but it's enough to cut down on your decryption by a factor of 16. A longer checksum is too revealing - even 8 bits identifies 1/256th of the crypto community, which isn't very anonymous.
Why not generate a key just for this conversation, and then post a full 128-bit (22 base64 characters) hash in the subject. You can even have a key for each message if the conconversation is two-way then whenever you are about to send a message you can generate a new key pair and include the new public key with your message. As soon as you receive and decrypt the message for that key, destroy the private key.