It will be required a very ugly hack (not easy either) in order to achieve that if Steem blockchain still hasn't support privacy at the core level. It's a very hard question we as devs are asking ourselves too.
I agree. It's a good question @demotruk, the Steem blockchain is not directly designed to do this (full transparency), I see some solutions like handling pgp encryption (would be almost a hack like aizensou said), or a mix in/offchain content (that can't get rewards) but i don't like that and as yet unclear..
It's of course a long term goal