[old_tv_nut]

I think I've reached the limit of my knowledge, especially without seeing the whole circuit.

Meanwhile,
1) How touchy is the brightness control? Does it have alot of excess range? This will give you an idea of how much the video signal swings compared to the brightness control voltage.
2) Can you cut off the blacks with the brightness control? If so, no adjustment to its voltage range is required, and you may just have to back off contrast (video drive p-p) to make sure bright pictures don't overload things.