I really hope Canon don't offer a fix for this, since seeing more test results.
It seems that this issue is only an issue at 0 or -1EV, which falls outside of the designed 2-20EV range of the meter. At these light levels, you should not be using the in-built meter anyway, so Canon does not have to act on something that does not fall within of the design specifications of this product.
I find myself in something of the same boat. Based on the growing body of testing I've been doing the meter appears to function properly in the specified range—and even some what below it—with pretty much anything lens you can throw at it without getting into teleconverters. And I seriously don't care for the precedent that recalling and "fixing" something that behaves correctly over the operating range it's supposed to work over sets. I honestly think the most appropriate solution would be similar to what they did with the 1D/s mk. 3 and Canon USA service notice dated 03-02-09; provide free service to those who believe their meter is functioning incorrectly.
I'm still writing up a report on this for my site, and I'll be sure to drop a link to the final version, but for the moment this is what my preliminary results look like.
The backlight becomes an issue when the light reaching the meter though the lens hits -6.5EV including transmission looses for the lens. In more practical terms for photographers it plays out something like this.
Lens Min Light Level Example exposure settings 24-70/2.8L or 70-200/2.8L IS II -3 30s f/2.8 ISO 200 24-104/4L IS - 1-2/3 25s f/4 ISO 100 70-300/4-5.6L IS @ 300mm 0 30s f/5.6 ISO 100 85/1.2L II -5 1.6s f/1.2 ISO 3200
As for leaking from a light shining on the top LCD instead of the backlight. I'm coming up with about 0.001% transmittance or something like 16-2/3 stops. So as long as you don't have some clown shining the equivalent of the sun at noon on your camera in the dark that shouldn't be a problem either. A light source that will light a surface 5mm away to 35K lx generates only a 1/3rd stop error when the meter sees ~ -4.67Ev from the scene (in my case that worked out to a scene lit to ~ -1-1/3Ev with an 24-70/2.8L lens on the camera).
It’s not that hard to make something light-proof. More seals. Walling off the LCD panel, etc. etc. I don’t design cameras, but just off the top of my head I can think of a few reliable ways of preventing light from getting through.
Incidentally, I can also think of ways this issue would make a difference: for example, shooting a dim room under one halogen light shining down on your camera could affect the exposure. If the LCD backlight can do it, so can an overhead light. I don’t know the details of how a light meter works and I’m not a physicist, but it I’m guessing that the amount of light a meter senses is additive. If the meter picks up ABC number of photons through the lens, the light leaking through the LCD display (or coming from the LCD backlight) will add XYZ number of photons to it. In well-lit environments, it may be a very small percentage of the total amount of light. But in less-than well-lit environments, it becomes a larger percentage of the total light hitting the meter and would make a bigger difference. I doubt the claims that it doesn’t affect the exposure when you’re above 1EV. People have already taken photos to show that it does.