In some of my thoughts about building systems on top of Subversion, I've thought about expressing workflow through custom properties, and acting on those in hooks. So instead of patches you'd have branches, and a submission would be a flag/property on the branch that said you were ready for someone to look at it. The "ready for someone to look at it" is almost an indexing operation as much as anything.
Though another possibility might be special conventions. For instance, it would be nice to encapsulate discussions of patches in the version control system as well. Maybe a file (discussion.txt or something) could store that information. This would be like pushing the Trac database into Subversion. I'm not sure if this would work with Subversion in practice, but that's the kind of feature that could make me switch.
That reminds me of Subversion hooks -- there's a lot of power there. I don't think that's transferable to a distributed system.
I believe projects that use distributed models have been using email for that type of message. i.e. "Subject: [MERGE] ab327e678c8"