The minute you do this, people will start complaining and (rightfully) demand that other IdP are also accepted.
So your system would turn from
cratename // global namespace
to
somegithuborg-cratename // your proposal
to
someotheridp-someorgatthatidp-cratename
in no time.
When we have arrived at that … what’s exactly the difference to doing the simple, obvious thing … and using domain names from the start?
I think it’s not that onerous to send a HTTP GET request to some host and check the reply.