You CAN encode a message in a currently accepted transaction using the broadband subliminal channel inherent to DSA (and ECDSA).
Googling for 'ECDSA broadband subliminal channel' to figure out what the heck you're talking about...
... doesn't that require that the recipient (as well as the sender) know the private key?
(you encode the message in the 'k' param, which the recipient can only recover using the private key?)