There's already a dedicated "source code" file type when submitting files to a project, so the "developer" side of things is essentially already covered. License information can be added through Dashboard --> Project page --> Metadata tab --> Release Info sidebar menu item. Not the most obvious place to look, but it's there, and it's displayed on the project page for players/modders/consumers if set.
For player/modders, though, there currently doesn't appear to be a way to filter search results by license (need to manually check the page). Only thing I could find is an "Open Source" tag. However, "Game Mods" is one of the categories you can upload a project as (there's currently 909 results at the time of writing), so I'd say there's already a streamlined way of making, publishing and attributing them...
I suppose your suggestion boils down to "make the things that already exist easier to find, and add an official tool to mark your project as modding-friendly"?