I think you may just need to cull the inner triangles.
Otherwise with the postprocess and the actor.
Enable custom dept stencil on the actor.
Modify a postprocess material to output the stencil in a gray color.
I think it would be solid.
You may be able to use a different pass to output the scene before the actor is drawn, so you can essentially make the stencil transparent.
not sure the material nodes to use offhand, but splitting or setting the character not to render in a specific pass should be an option in the same area you enable the stencil.