Bunsen, your mixed RGB circuit should be the best choice. You want mostly green in a grey-scale display, then some red and less blue. Second choice is just to hook the video input to the green output. Another source of problems might be the sync lines. Some monitors use composite sync on green, some use separate sync, etc. If one of the sync outputs is grounded, that might prevent video from working, but I'm guessing about that. I think you're just going to have to use Ohmmeter and trial and error. There's not much voltage or current in video outputs (ADC excepted!) therefore not much chance of BIG smoke.