Steemit can handle that kind of scaling. Is it a high value post though? A transaction fee would be required.
There are other ways too, like break the private key up so it's owned by a group and then let them vote to decrypt it in some fashion and then it's posted in a self destructing note page. There are ways if you really want to do it, or just look at Bitmessage which managed to do a decentralized encrypted 4chan type thing.