Cast shadow on invisible mesh

I am building a game that uses prerendered static backgrounds similar to classic RE games. I have the cameras and scene working fine. I cannot figure out how I can get the shadow of the player character in the world.

Here is a screenshot of what a scene looks like:

Here it is with the collision planes which have the shadows (the collision planes are hidden during gameplay):

A typical render flow for this scene involves:

  1. Get the depth map of the scene
  2. Compare with the collision data depth map
  3. Render pixels based on priority of above depth maps

My question is, how can I extrapolate the shadow map from the collision planes, and then add them back in the post post process material? So basically, can I have an invisible collision mesh that can also have shadow casted on it?