If I use my handheld spot meter I know I am metering through a 1 degree field of view.
In my cameras a spot meter is much wider than this and varies with each camera.
I wondered if it would be possible for CHDK to have a means of metering the centre spot of the sensor at a given angular value, eg 1 degree.
I can do that after the shot and return a Bv for any specified area of the picture. You can then use this value to take another shot.
Pre-shot metering isn't available yet. There's a pre_shot live histogram available, but that uses the full picture area. It might be possible to turn the pre-shot histogram into a Bv value. I'm not sure how accurate it would be, or how hard it would be to program.
But if you're willing to throw away one shot, and use the 2nd one, I should have something ready soon.