Two cents: The Basedir Spec, used by free software desktop environments like Xfce, LXDE, Rox, GNOME or KDE, proposes a scheme that would fit the bill:
I proposed following the spec in #7175 too.
There exists a Python module to retrieve these dirs, but it’s poorly written. I’ve been meaning to get in touch with the maintainers and propose improvements for a while but haven’t had time yet.
A very nice thing in my opinion would be to integrate config file and command line parsing. Logilab and others did that. Countless projects would benefit.