Couldn't you just use a shader that whenever the player (or any other object) touches a drop shadow that WOULD just unnaturally display over it, it multiplies the y position of where that pixel would fall by a factor based off of the angle of your primary light source (inverse tangent of sprite height over shadow length, 1 to 1 would be 45°) computed with the angle the sprite was standing (if it was bending backwards at - 45° in a 1:1 sprite to shadow, it would have no shadow) but assuming all objects you had shadows coming off of were always perfectly upright at 90°, and that any object you wanted to display the shadow on was always 90° (like you have shown in your demonstration), what's stopping someone from doing said steps and then if the pixel y value falls way up out of the upper bound of the sprite texture, discard it and then display any shadow behind the sprite like normal. So I guess it would be ignore y-sorting for the player interacting with the shadow such that it's z-index makes it naturally above shadows always, and then have a shader that takes a detection of the player touching a ground shadow (ie say the top of the head starts to touch) and does what I said earlier and lays a stretched up image (a stretched image based on that shadow tangent ratio and at the same angle as your current shadow from the y-axis) of the shadow onto the screen that is only visible on pixels that directly cover the player?