Simply use free Usenet Servers via Tor and post your anonymous* PGP
messages to the Usenet group alt.anonymous.messages.
*You send PGP encrypted messages via Mixmaster Remailers (via Tor) and
preferably use a hashed subject (hsub) so that the intended receiver(s)
can filter out their messages.
Have many servers who just accumulate encrypted packets and then only the host knows how to reassemble them in order and decrypt.
Could probably be somewhat realtime if executed correctly.