This is a good idea. It seems like everyone working on this problem so far has been trying to start from scratch (Diaspora, Tent, etc), but building on existing popular platforms would give a system a huge advantage.
I don't know exactly what it would look like, but I think it's worth exploring.
Some sort of WP (or other software) plugins for a messaging/activity stream, and a distributed pubsub messaging system between sites? (using probably json over http/spdy/websockets, not something separate like AMPQ)
It would require certain capabilities from the participating blogs/pages/webservers, and it would be fragile in the case of url changes, but it might be workable.
http://buddypress.org/ is the WordPress-based solution for messaging/activity, but it's mainly intended for niche social networks. To fully build a distributed and decentralised system, we need both a protocol to communicate between servers, in addition to the support from those servers.
"both a protocol to communicate between servers, in addition to the support from those servers"
Yes, I think this is the advantage of WordPress over Diaspora. To some extent the infrastructure is already in place. Diaspora runs Ruby so ubiquity might take longer. (I was skeptical of Diaspora at first but I do think it has a fighting chance.) I don't have stats handy but I'd guesstimate there's several million self-hosted WordPress blogs out there representing tons of computing power, at least a starting point. Why bet on a Twitter or Facebook or Google Plus API that could be shut off without warning when you know WordPress is here to stay? I've been saying this for years--since then Automattic has launched Jetpack and user profiles, extending Gravatar.com, but we're still a long way off. But as I've said before, I think something like this would need Matt's blessing, not because he's a gatekeeper but because I think it's an ambitious idea that needs an influential visionary behind it to pave the way forward. Anyway, I'm not saying anything new, lots of smart people are working on decentralized networks.
I don't know exactly what it would look like, but I think it's worth exploring.