Missing the AF_BUS

Yep. You can even use the dbus protocol over that socketpair. libdbus could probably provide some nice convenience API to facilitate this where you'd basically get a DBusConnection back from a method call to the remote service.

The downside is mostly that it's a fair bit more work for apps to do stuff like this. Services don't necessarily need to track "registered clients" right now but with this kind of setup they have to, in addition to dealing with the raw sockets and other extra work.

A lot of the discussion of speeding up dbus is motivated by trying to make the easy thing work well for apps, instead of requiring app authors to sort out these tradeoffs.

Especially with the higher-level API in say glib's dbus support, though, it might be possible to near-automatically put certain objects on dedicated sockets. Just a matter of programming...