Since WhatsApp is end-to-end encrypted you can probably just send bibary data. Stick a prefix on it so that the real client is guaranteed to ignore it as corrupted.

I think the only risk is that if you have a real client running it reports the invalid messages and WhatsApp uses this as a signal to van your account.

Note AFAIK WA is e2e encrypted BUT they can flag any weird looking messages (weird patterns etc) to see and review their contents.

So I think Meta/WA can opt to decrypt any suspicious messages they come across.

How would that work? Do they send a request to your phone to decrypt it for them?

I'd imagine the app just flags suspicious messages and sends them directly to review, in parallel to sending the encrypted message to the receiver

Meta/Facebook is the last company I would trust regarding their E2EE. They probably have a key themselves.

The E2EE here is not about privacy, but about being able to send whatever data you want (like binary) since WhatsApp will only see one type of data (encrypted) in transit, in contrast to needing to send data in a specific format to have it transferred at all. Meta can peek at the original "messages" all they want, they will see encrypted packet data anyway.