I have been contemplating how to mark broadcast messages as being 'for' someone. To foil traffic analysis, you don't want to include their nym or key-id, for the sake of the your poor CPU, you want to avoid the need to attempt decryption on everything that passes through.
The main problem is how to avoid decrypting _most_ of the traffic, without giving away significant information about the recipient. One approach is to do something some political users have been asking for - implement support for very short keyids (e.g. 4 bits instead of 24-32), so that the keyid isn't a good identifier for the user. Another approach is to include a tag in the Subject: with either a hash of the key (substantially reducing the number of bits), or simply the last hex or two of the keyid - that lets you ignore 15/16th or 255/256th of the traffic, without giving away much.