You'd have to send them two messages of 0.001 SBD:
(1) "I encrypted my message with your public posting key. Use your private key to decrypt it"
(2) "ADFPOIUNSDFOIWNUNSUDOPA ..." (encrypted message)
Steemit dev's could implement this, and they could implement it without SBDs involved.
RE: Private messaging in Steemit is already here!