Thank you, great explanation. So getting back to the original comment on this thread, I still don't get where the question comes from.

If hivemind writes a custom_json to the blockchain, without any interpretation, then as long as you have access to the hivemind code (for example, if it's open-source), you can use that code and read the hive blockchain to re-interpret those custom-jsons and verify everything on your own.

So, every HAF app will be writing custom_jsons on hive, and if the HAF app code is open-sourced, other apps can communicate with that app, given the data is now public.

It just needs some extra work compared to actual writing.