I have to admit, I am having trouble imagining how an MSY equivalent would work with Witch rules still in place, even though its a very cool idea.
The main problem I can see, other than Kyuubey possibly having a motivation and ability to disband somehow through some convoluted plot due to it being an entrenched entity maybe, is that unlike wraiths, witches inherently only come from magical girls, so sustainability caps out faster, and is dependant on some level on failing to save some people, with success of a worldwide scale reducing resources which reduce the cap and so on with girls still needing the resources. And with no godlike Madoka wish...
I do think a global group is a worthwhile idea, but I don't know how universal, or long-term it could actually be.
Though maybe this is pessimistic or a limit on my imagination. It is interesting to discuss in any case.
Edit: I also have to say, the idea that Yui is only contracted in a Witch based universe and nor in a relatively benign one, is kind of tragic and sad to me, as she would be much happier that way as I understand her character.