I personally think Hx in the Mu setting is already dealt with largely in the way that a lot of games have people who try to connect their characters to each other prior. This would simply be a more direct and codified way to do it. As for implementation, I'm sure someone with more spare mental time than me could come up with a way. Off the top of my head I'd lean towards histories perhaps being a bit reworked from RAW and to allow them ebb and flow a bit more as incidents on game occurred (as they're supposed to in AW).
As for your scenario above, I would presume the Story Teller would be the person implementing such things.
In the DW game I have been a part of in the past, the story teller rotates every week or two depending on the mood and flavor; much like a MU. So I don't really see how that's an issue in the least? That's sort of how most games I have seen already do it. Someone sets up event, runs said event, they get to decide such things in said event?