I did an RGB mod on a Trinitron, but it was a few years ago so the details are fuzzy. However, I didn't use a switch like I see a lot of people do, but instead made a simple circuit to mix the existing OSD signal with the injected RGB and it works great. So volume and whatnot is displayed as you expect. Since sync is on a different line, it seems as though you should be able to adjust the rgb signals on the inputs so you wouldn't have to mess with the flyback, but I'm not 100% sure. Try looking up video attenuation schematic. Or video brightness schematic.
I'm not sure about using the existing inputs in conjunction with the rgb, since mine is in an arcade cabinet and only used for that, but I would think that if there's no RBG signal, then it should just display... since the rgb is basically being shown as an overlay.
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