warnings._processoptions is called very early, before site-packages are enabled. Because of this, using a non-standard 'category' will almost certainly fail with the message:

    Invalid -W option ignored: invalid module name: '...'

The -W option would be a lot more useful if it could actually match non-standard categories (it does, after all, pretend to support modulename.classname).

I don't see any easy way of fixing this, other than initializing the warnings module later or matching category names with the given string (and getting rid of the import).