Not really that I can think of--in order to register the connectors the modules defining them would have to be imported somehow.  I suppose in principle you could provide a list of these somewhere else, but ultimately the list of modules that need to be imported for all the connectors to work needs to be provided somewhere.

I suppose one thing we *could* do is in a central place list what connectors are supported--in other words, pre-fill the registry with information like "tables can be stored as fits files using the connector in `astropy.io.fits.connect`" and that module would only be imported if and when someone tries to read/write a table in a FITS file.  On some level that makes the metaclass-based auto-registry irrelevant though (although it could be useful for seamlessly supporting third-party connectors?)