The use case would be for testing incompatible and/or major changes. I'm currently working on implementing a new eclass in its own topic branch rather than in the master branch of my overlay as I don't want anyone depending on something else to be subjected to incomplete or broken changes. When setting up another machine for testing, I'd like to be able to specify using that particular branch without having to do a lot of fancy footwork.
Yes please, I would find this useful too.
