When using RGB the Xbox is limited to outputting 480i, which is the reason why I want to know if there is a visual difference between YPbPr and RGBHV from the Xbox.
When I say “RGB”, I don’t specifically mean RGBS (sync-on-composite) from a stock system using a SCART cable, which is indeed limited to 15kHz. Assuming proper shielding, it doesn’t matter whether sync is separate, composite, or muxed on green–RGB is RGB.
What I was saying is that you could use a modified BIOS that forces the console to output RGsB full-time, which would get you RGB over regular Xbox component cables, which you can connect directly to your OSSC (AV2-RGsB). If you get a 3x RCA to DE-15 adapter (Or a custom, componentless Xbox->DE-15 cable), you could connect the Xbox to an Extron RGB interface, use a 4x BNC to SCART adapter/cable to connect that to your CRT, and then use a DE-15 to component cable/adapter to connect the loop-out to the OSSC. That would let you have both displays connected simultaneously, and you’d only need to shut off the CRT when using a video mode it doesn’t support.
(Other approaches are available, too, such as using a component video splitter to split video before the RGB interface, which might save you a little money on cables.)