What I am using is able to function with a dust collector, compressor and the spindle itself active. Which is a VFD.
So in this case it would listen for a specific frequency. One that would cut thru the chamber noise. And hypothetically if glowforge would support such a thing overall end user usability would be better and safer. Versus another target that would be registered and can also be a potential obstacle and fire hazard.
Thou using a NPN style sensor would be easier. I dunno if it would be the right way to go.
And on top of all of that- This is all in theory. What i posted above is just a simple human powered method of doing round objects that anyone can do. And can be built using the glowforge itself and a couple bearings and M5 bolts.