[eluser]marcoss[/eluser]
In the first place, and from an design perspective, it would be better to use domain.com/rss/username instead, otherwise, when your app grows, routing will get really complex, you will have to deal with domain.com/user/username/photos, domain.com/user/username/notes, domain.com/user/username/blog, all in the same controller, instead of photos/username, notes/username, blogs/username.
Anyway, if you still want to go with that design, you should avoid the routes option and use the _remap() function inside your controller, there you can intercept the request before the default method gets called and route it based on the username and "action" requested.