Hi Steve.
I really appreciate all the feedback. Keep doing that. It gives me a lot of ideas I didn't think about.
I implemented the moustache by creating a HTML select with three options. 1 full message object. 2 selectable object and 3 moustache.
Depending on the selection I show different input fields to the user.
From a user standpoint I think this is the most easy to understand instead of explaining to use a standard text field.