I really don't think its practical. For one thing, most of the time a variable amount of light is leaking thru the viewfinder as well, and the camera does not know where its coming from.
Its really a non issue, more of a nitpick that Canon has apparently decided to address for those who see it as a issue.
Most cameras leak light through the viewfinder, at least my 7D and nikon D40 do this. light bounces of the matte focusing screen and into the meter. Thats TTL metering for you. And btw canon include a viewfinder cap.
Still don't see why it would't work for most users.
The amount of light leaking varies. The ambient light will vary, as well as the top LCD light is not going to be the same brightness on all bodies. And those using flashlights to read the exposure are going to leak totally different amounts of light to the meter. How could they possibly pick a number, even if you did not have light leaking thru the viewfinder?
I feel that i might have answered som of your questions already, but i'll try to step up my thinking and explain.
And different batches might leak differently. That shouldn't be a problem since the canon firmware can identify the cameras serial number and adjust the behind the scenes exposure compensation accordingly.
This was me trying to say that different cameras will have different light leak characteristics. But I think all the cameras in one production batch should be very simmilar, and canon should know wich batch (semi-big number of cameras) has wich serial numbers.
If the meter says < 0 EV and the camera knows the LCD is on simply add say -1 EV.
Or of the meter is giving 0 EV then -2/3 EV, if 1 or 2 EV then -1/3 Ev.
I don't know the numbers, i'm just guessing.
I'm trying to say that if the ambient light is X EV dark and the cameras computer is told that it's LCD is on it might put one and two together and fix the exposure according to the serial number. Since metering 1EV with LCD ON is equal to metering say 1/3 EV without LCD ON on some batch it might subtract 2/3 of an EV in that case.
And now that turning on the LCD won't bias your exposure why would you use a flashlight? But you are right, those users are sqrewed, unless they remember how much their flashlight messes up their exposure at different EVs and adjust accordingly. I really am not being sarcastic here.
I'm not saying that this would solve all the issues, but if the hardware allows firmware to add exposure compensation behind the scenes - wich might not be possible. Most users that think they kinda might sometime want to use their camera in the dark this way wouldn't need to choose between
a) Loosing their camera for a couple weeks while some canon worker puts tape on the back of an LCD
b) have to live with the fear of the LCD screwing up their auto exposure, maybe, sometime, if it's really dark.
c) Install new firmware (1/2h tops) and just try avoiding flashlights really close to your LCD (minor inconvinience for most).
But the real question is whether the firmware knows about the LCD and even has this level of control over the meter.
Wow - it's late. Maybe i'm not thinking straight.