Well after I made my last posting I realized there is a mush better way to avoid having identical reply adresses: Simply let pgp make a new encryption of the return adress. This works because pgp uses a different key for each message, and the key is the only information that is encrypted with th RSA algorithm. In other words if you encrypt a message once with a key and encrypt the same message again with the same key, the resulting ciphertext will be totally different, with exeption of the first bytes and the length of the text.