can we add an overlay on top of the sides of the screen, so that even on a widescreen display, it hides anything that goes over the square in the middle of the screen?
The following is a circle.
you could use a Babylon layer for this 
https://www.babylonjs-playground.com/#08A2BS#1 the last parameter is changing from background to foreground.
If you only rely on a vignette effect you can also enable it on any standardMat or PBRMat with:
scene.imageProcessingConfiguration.vignetteEnabled = true;
and configure with all properties:
scene.imageProcessingConfiguration.vignetteXXX
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May you show a demo for me 