As I said in the article, many years ago I played with using XMPP as the side channel for NAT traversal software. XMPP was handy for this because you can just define your own extension that chat programs will ignore, but XMPP itself will still carry those extensions end to end. The idea I had at the time was to have the p2p software signed into your XMPP account, and it would use that as a federated message bus between all your devices, and the devices of your friends.
Honestly, the idea only made sense back when Google Talk ruled the earth, because it gave XMPP a network effect I could utilize. Sadly, since then, chat has fallen into a dozen different silos, so the "piggybacking" benefits are zero. And if you're not piggybacking on an established network, you can build a side channel protocol that's much simpler than what you'd make with XMPP.
If I were trying this again today, I'd probably poke at what Matrix can do in terms of non-user-visible messages. It seems to be a successor to XMPP in terms of the communication model, so if it can do broadcast+unicast of non-visible messages, and supports sending to yourself as well as others, it might be a good modern replacement!
Honestly, the idea only made sense back when Google Talk ruled the earth, because it gave XMPP a network effect I could utilize. Sadly, since then, chat has fallen into a dozen different silos, so the "piggybacking" benefits are zero. And if you're not piggybacking on an established network, you can build a side channel protocol that's much simpler than what you'd make with XMPP.