I’ve had a look at your implementation.  It is an interesting use of properties, but doesn’t quite fit into how packaging works.  Most of the options parsing and validation (from config files and command-line alike) is done in each command’s finalize_options method, some parsing and validation is done in the config module (for the metadata, files and soon extension sections), and the section named “global” is validated by the Distribution (or maybe Dispatcher now) class.  So I’ll certainly reuse your tests, but will have to redo the implementation.

I’m also wondering if install_data’s options are the right place for this.  For example, does it make sense to allow “pysetup run install_data --categories spam=blah”?  (Any command option can be given on the command line.)  Maybe the global section of setup.cfg would be better.