devkitPro

I am requesting an example for rendering 3D objects over a 2D background utilizing a pre-existing depth map, ideally done mostly in GPU+citro3d as with the other graphics examples. I am quite open to a "No, this is dumb because XXXXX" response, because I'm not even sure this is possible with the 3DS's GPU. Then again, I am quite dumb and pretty certain I have no clue what I am talking about.
For a visual representation of the kind of example I am asking for:

• Prerendered background:
  
• Corresponding precomputed depth map data:
  
• 3D character culled by depth map and rendered in foreground:
  

My sincerest apologies if this is an unreasonable request.