That is the thing. I know how to turn shadows on and off so I thought I could answer this but it wont work if the mesh itself is not visible.
Best thing my mind could come up with is if you don’t care if the Ai is actually there to cast said shadow then I would just make a black decal with default settings as it would give the illusion of a shadow in the shape of your Ai Char and then spawn it and De-spawn it as needed giving you the illusion of a shadow. Might even be able to animate it if your good with materials and animations.
Let me know if this will work for you and I can guide you further.