@matrix I'm a bit perplex that you're planning to implement WhatsApp's proposed solution. WhatsApp is obviously trying to dictate the messaging protocol, so they have it simple and easy while everyone else needs to do the hard work. This gives them a massive advantage: They can make changes to their liking, iterate fast on a single protocol while everyone else needs to juggle two (potentially totally different) protocols, most likely with significantly less available resources. The more messenger solutions join in, the more likely it is that it will become the de facto interop standard.
Whether you're a full on interop hardliner or have your doubts like me, I was hoping we would be united in one thing: WhatsApp's proposal can't possibly be in our interest.