Hey Chosker -
I have not tested your exact methodology but you could try to adjust the SceneCapture2D Actor’s Max View Distance Override to set your ground level and then use that distance to divide by the Scene Depth in the Post Process Blendable to get a range and then you would probably want to normalize it to between 0 to 1 (though if I am not mistaken Scene Depth already normalizes, so you may only have to Clamp it).
Eric Ketchum
EDIT: You will probably also have to layer several render targets together to get a full un-distorted view.