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.
Hello and welcome,
I am a new to this community,
I try to find my way up and increase the numbers of followers
I already follow now, please follow me back @samuraiz
Good luck