Yes, the issue is that the red illumination in the 5D3 affects the metering when both are on at the same time. In the 1DX they were able to alternate metering with illumination but on the 5D3 this was not possible with just a firmware tweak. That is a fact that Canon has already announced. People frequently miss the point mainly because Canon doesn't talk much about problems they cant fix. However this one was discussed by a Canon representative. It was looked at and deemed not possible at that time with just a firmware solution.
Hopefully the redesigned 7DII will incorporate these fixes and we will likely see something whenever/if ever a 5DIV gets to the drawing board.
With that said, it's been about 6 months and I'm sure the smart firmware engineers have not dropped it but continue to look for ways to make it happen.
I doubt that will happen, ever, due to the 5D3 design. In order for it to happen they have to take away metering speed. They already stated that the red AF points affect the metering and they dont have a way to deal with that in firmware.Maybe I'm missing the point completely (wouldn't be the first time by a long shot), but illumination of the active AF points in red is already provided (Canon 5DIII instruction manual, page 103: VF display illumination).
Hope they fix the AF points so that the active point is red
It's true that when this function is set to either Enable (always) or Auto (only in low ambient light), the duration of the illumination is quite brief (1/4 second or so), but -- according to my VF display -- the metering is achieved (virtually) simultaneously with the illumination of the active AF points, so I don't get the "affects metering" argument.
It seems to me that a firmware update could be easily implemented to select the duration of the AF point illumination; i.e.: the brief duration currently available; 1, 2, 4, 8 seconds (just throwing out possible values here); or as long as the shutter button is depressed halfway.
Again, if I'm missing the point entirely here, I would love to be schooled. I do appreciate that metering can remain active as long as the shutter button is depressed halfway, so as to modify meter readings in changing ambient light levels during composition (i.e., passing clouds or focus/recompose). But it seems to me that an algorithm could be developed to take AF point illumination into consideration in the metering calculation, if / when the light spill from the illuminated AF points does impact metering; something like: adjust metering by x% when (count of AF points illuminated x light emitted per illuminated AF point) > y% of ambient light level of metered scene.