Cast shadow on a transparent object

Hi
I am trying to add some 3D objects (fonts) on a video (a wall, with fixed camera position), the fonts should cast a bit of shadow/AO to the wall on the video so ideally I d need to cast shadows on a transparent plane.
Another way would be simply to use a dropshadow on texture but I d like to retain the MRE render passes to achieve some reality.

S.