Setup DepthStencil for SceneCapture2D? [Portals]

I’m currently working on a portal door system where I got some SceneCapture2Ds moving and rotating around portal doors while facing them and some material alignment tricks to get the perspective right.
The effect is looking really good, but I got some serious performance issues, because rendering space around the portal doors is wasted, especially when the camera is a few meters away from the door.
So I thought about setting up stencils on the portal door planes and only render the scene inside the stenciled areas.

I already inspected the SceneCaptureRendering.cpp, but I don’t know how and where to setup the stencil for drawing properly?
Could someone please help me?

ps. My portal doors are shaped like real doors (rectangles e.g. 1.0m by 2.2m)

Anyone? EPIC?

I am trying to do the same thing. Any luck?