I decided to buy one of the glass protectors for my 5D MK III due to the integrated lcd / cover.
The new one is made of glass, fits fine, and doesn't seem to cause a issue with light.
HOWEVER - It shows oil, moisture, and finger marks something horrible. The oils bead up on it and that does bother me. I've cleaned it with eyeglass cleaner, and that helped, but it is not as good as going without.
For now, I'll keep cleaning it to see if it looses whatever coating is causing the issue. Its like my car after a wax job, the moisture beads up on it.