That is not the issue, please have a look at the playground in my previous thread:
Babylon.js Playground (the whole setup)
Babylon.js Playground (just the baublesScene)
Before I asked this, I did manage to put a plane with a RenderTargetTexture right in front of the first scene’s camera, like you are suggesting. But the texture was wrong (due to the “synchronized” camera) just like in the first playground. In the second playground you can see what the overlay (the baublesScene in my case) is actually supposed to look like. In the same thread @Deltakosh confirmed that Babylon.js indeed does not support what I am trying to achieve there.
The reason I needed two separate scenes back then was to control the lighting separately.
Now, the reason is that I want a clipped area that does not span the whole screen, as described in the OP.