Filament transparent shadow catcher ground plane. (SOLVED)

FrankTheTank

I know how to make shadows in Filament traditionally, but there are times when i want to make a looped animation, of say a character running in place and I don't want to render the ground, I just want to render the figure and the shadow, and then I can add that loop over a moving environment later. But I can't figure out how to make a  transparent shadow catcher. Has anyone figured this out or is it not possible yet in Filament?

 

Update: I solved it with a "cheat" by using flattened geoshells.