I made some research to try and understand how these demos work and how we could maybe use it in BabylonJS.
Here is a list of app/demo I have found (I advise you to open one link at a time 
):
Only Spheres and Cubes:
If you look at the Github project and shader code, we can see that they are made to render only spheres and cubes so not sure we could use them.
Full Scene:
- Made by Katendal - Github
- Made by Lighttracer
- Made by Dassault - Github
- Made by Eric/ThreeJS - Github
Therewith the GitHub projects, it becomes harder for me to understand how it works 
. But as they manage to make path tracing rendered with an environment texture and a gltf object, I guess it means it would be possible to actually do that in BabylonJS?
I think the ThreeJS use case is interesting also as it seems that Eric managed to have Path-tracing that can be applied to any ThreeJS scene.
Other WebGL/Path-tracing projects found: