The mags have a spring loaded tension button on the side and thus is one of the easiest mags to load in volume. Its one of the more enjoyable aspects of using a GSG STG 44?
To answer your question a McFadden Machine Lightnin' Grip Loader with a modified AR mag adapter seems to be possible. There is a YouTube video of the subject and some threads on line from another firearms forum. After that, its a sore thumb.
Candocad.