That is indeed incredbible, my graduation thesis was porting my supervisors' particle engine from Objective-C/OpenGL into C++/OpenGL/MFC, because his NeXT Cube was going to be decommissioned. This was still during the 1990's.
I have a non-working Indigo 2 10k that I keep meaning to refurbish, but finding a monitor that supports sync on green that still works is exceedingly difficult.
I wonder if you can convince a dirt-cheap GBS8200 with GBS-Control mod, to convert sync-on-green to VGA. I'm not sure what options are available for pure sync extraction, since the GBS-Control also does color conversion, deinterlacing, rescaling frames to more or less lines, simulated scanlines on LCD displays, etc.
IIRC many Samsung, EIZO screens from recent past support SOG.
I heartily recommend EIZO LCDs. They are awesome. Support everything. And you can fine tune everything including clocks from their onboard settings.