For vanity GPG keys, I came up with a neat trick where you could insert arbitrary ASCII art into your key. You construct your ASCII art using the Base64 character set. The decoded binary will be gibberish but you can place it in an inert Literal Data Packet that will be ignored by any conformant OpenPGP parser. It just takes a little finagling to align your data to a 3-byte boundary with appropriate line wrapping.