Check the documentation for "custom_json" ;) you can store anything you want.. In fact the "follow" feature is already implemented not as a native feature but as a custom json tx ;) check here I encoded the Bitcoin Price back a few months ago in a custom json block just to prove the point (I just made a new one since I don't feel like digging back through all the blocks to find which it is in). You can see the custom_json containing today's bitcoin price in a custom_json tx encoded on blockchain!

https://steemd.com/tx/3a4bc6b8b74db88ae269dfaa5938c5b192143f29