Indeed, I'm surprised it's taken this long for people to take advantage of bittorrent DHT for other p2p stuff.
You don't even need to change the protocol to start using it. It's a ready made hash-to-peerlist mapping, so you could just use it for peer-discovery/bootstrapping and do other stuff out-of-band.
There already was a BEP to exchange arbitrary stuff, BEP44, for mutable or immutable items. It's taken this long to use this BEP specifically for mutable torrents, though, because it turned out that not many people had a need for mutable torrents.
Is this BEP44 actually implemented in most clients? I was experimenting with p2p some years ago, and I considered extending the protocol, but I figured it would be more practical to work out-of-band, rather than relying on widespread implementation of an extension (which didn't seem that likely to me at the time).
You don't even need to change the protocol to start using it. It's a ready made hash-to-peerlist mapping, so you could just use it for peer-discovery/bootstrapping and do other stuff out-of-band.