So I [in theory, I don't know how to start with this on a technical level] could make a third-party Signal-compatible app, but allow it to connect to Whatsapp instead of Signal? Even if I can't use my Signal account to contact Whatsapp people, that's still potentially useful. Although I imagine the terms I'd have to agree to to do so would be full of nonsense that stops this being remotely feasible.
I guess I'm misunderstanding here - I thought Whatsapp would be the "service" in my case, I'm just making a client to hook into their, presumably open [to people who agree to whatever their terms are] API. So it's more of a federation thing between services?