If you just wanted to store it, and only you know its there. Then here is one way to do it.
Make a post, as you said, with base64 encoding or PGP.
Then after a blocks time has gone by, edit the post and paste something for everyone to see.
You can then go to steemd and see your post, but basically, you will have to know what you are looking for.

I am pretty sure you do not edit the original block created, so its always there. (I could be wrong)