I wonder if a keyboard app could do it, since they sit between the user input and the chat app.
It would be nice if message transportation were decoupled from composition and consumption. Default bundling is fine for ease of use, but allow first-class replacements.
These are called in-line encryption systems. They're generally not apps, but separate devices with automated ciphertext transmission. I've been working on something that does this http://github.com/maqp/tfc and that can be plugged to almost any transport system with relative ease. The current design is using v3 onion services for each endpoint.
It would be nice if message transportation were decoupled from composition and consumption. Default bundling is fine for ease of use, but allow first-class replacements.