Actually I've been writing a new feature on my Mechanics module that could do this. The feature is called Plumbing and it provides a replacement routing system that can be configured in a variety of ways.
It's not quite production ready yet; the latest source code basically works, but there are a number of known issues.
Something to note about Orchard is that users are actually just stored as content items. My Plumbing feature then allows you to flexibly customise URL building for content items, and then have sub-URLs based on connections between content. The main feature
of Mechanics is defining those connections. So with perhaps a bit of extra configuration and customisation you could definitely implement what you're describing. You just need to create a connector from the user to the content, each time the user creates something
- this will probably be a standard feature eventually, but you could do it easily by hooking into the right events. Then finally you'd have those connectors control URL generation.
I think doing it otherwise by hand would be quite tricky and you'd end up having to reimplement a number of Orchard's core mechanisms if you still wanted your users to have all of the existing CMS features available. Whereas with the approach I've described,
each time you added a new content type it would just be available to all the users without having to write any new routing code.