I highly recommend adding the merge request workflow into Codebase, similar to Github, Gitorious, etc.
Our team is considering moving to a local install of Gitorious just for this feature.
We've implemented this suggestion!
This would be an awesome feature! Something I really miss in Codebase as I used this a lot on github.
In order to have a pull-request style feature, it would be helpful if we could set permissions per-repository and allow merge-requests from different repositories