And what about the Parental Controls that MS already introduced?
Maybe they don't (yet) have the granularity to just flag DLC and not the whole game? TBH, I haven't had much reason to look into the parental control features so I'm not really able to speculate on the mechanism and whether it could be used as you describe and if not how easy it would be to extend it so that it could.
Do you think that would be enough to get MS off the hook? If so, that's probably what they should do, ultimately.