Perhaps you can circumvent the 20 sec limitation by filling posts with more than just one post and storing the information in a buffer until it gets written into the blockchain. All you need is a page separator and then you can pretend the new post begins. It wouldn't really be compatible with Steemit because of the visuals, but for example if you build a chat, users could create an extra account with a letter combination before their username like @chat-edicted and then the app saves one post every minute for all messages sent by this user and also the same with his conversation partner @chat-doodlebear. To every message in the post you add references to the partial posts by adding the time stamp and a reference what and from whom came before and what after that specific partial post aka chat text follows. All you have to do is to teach your app how to access the right posts to show the conversation in the right sequence.

Or, you can do it via the wallet, which is probably the better way because if you add a # at the beginning of the memo, it will be invisible for others. Only the recipient can read it. The downside of the wallet method is that images and video cannot be added. For that, you could use the posting function, although, that content would be visible. A partial fix for that would be to anonymize this part and create one account for all image and video content. Although, that wouldn't change that everyone can see the pics. All they'd have to do is to look up the content of this one for all account.

Leaving away images and videos would be a loss, but creating a blockchain based chat app would surely be something.