Question Camera Stacking

What I am after is the ability to layer a number of cameras and then render each layer (camera) separately, blitting the result to the previous layer after I have processed that layer.

What seems to happen is that the previous layers are fed to the next camera to draw over so I never get a render of the objects on that layer.

What would be good I guess is if I could render each overlay camera to a RenderTexture but this does not seem possible, the option is removed for overlay cameras. Stacking a bunch of render textures without camera stacking means I cannot use post processing as it makes the render textures lose their transparency.

RenderObjects would maybe seem the way to go, but this seems to want to apply a material to the objects on the layer being rendered which isnt exactly what I need.

Can someone point me in the right direction please.