To make it work, you'd have to redesign the magazine, which would then "obsolete" jillions of existing HK and contract mags (not a good thing). This is the exact same issue with the Colt 9mm systems vs. every other 9mm clone AR. The Colt mags have a tang on the follower, which pushes up the bolt catch when the last round is chambered. To facilitate the bolt catch, the Colt mags have a groove down the back "spine" of the mag. This is why Uzi mags will not fit in a standard Colt receiver and vice versa.
The HK mags would have to be redesigned with such a provision, so the "new" mags would not fit the old guns, or the old mags would not activate the bolt catch. Either way, with the UMP replacing the MP series guns pretty much, it's doubtful we'll ever see such a thing come to pass.