An aside but this is giving me ClearCase config_spec vibes.

Memory is hazy - possibly due to trauma - but you could create a config_spec which would make your view (roughly: working tree) pick up different bits of the filesystem from different branches. Branches might only exist on some of the filesystem, depending on how you set up auto branching and what you checked out, and you had to know what the config_spec rules everyone was working to were or the whole repository was effectively broken.

It was very complicated to get your head around - I guess not helped by the codebase I was working on at the time which tended to concentrate change in the same 20 files.