Petr, it's what happens after you've worked on your modification in private and you decide to publish it to the world that's the interesting question.
You can make a patch and post it to Bugzilla, or you can publish your repo on your own server where the maintainers can pull from. Bugzilla isn't as attractive as some kind of anonymous micro-branch, which after all is basically what your patch is. Publishing your own repo is not stable over time. In reality, patches often wait many months for integration, and private repos are likely to move or go down, and you still have the problem of tracking the links to all the private repos.
It seems that the problem with turning anonymous patches into anonymous branches is that svn can't merge between repos (e.g. main and pending-patches), and people are uncomfortable adding anon branches to the main svn repo. Right?
Well, I argue that centralized version control systems ain't any better than decentralized ones here. You can make server-side anonymous branch with sensible decentralized VCSes - at least git and monotone can do that, I'm not that familiar with the others; but you can always just have parallel repositories at the server.
So this is NOT a problem with distributed VCSes.