Clan rankings could be done through a tournament which would be held by only by the most reliable people since people who have tried setting up a tournament have failed because of bad organisation e.g. TISC tournament.
OR (just a suggestion)
Have a couple of dedicated servers which would only be used in scrims and wins/losses and draws would be recorded somewhere, maybe see-able in forums aswell. I don't actually know if it's possible or not.
Alternative:
Just do it normally, scrim in a server, screenshots and demos provided. results recorded in some sort of clan ranking chart.