Are you covered if you use Magic Lantern?
DIY Photography did a great article on third party firmware and whether or not it voids your warranty. They did it for various brands including Canon, Nikon and Panasonic. Of the three, only Canon is “okay” with you using third party firmware and your warranty will not be “voided”. The only catch is if you “brick” the camera and it’s related to third party software, Canon will not cover that repair.
“There is no such thing as “voiding” the Canon warranty, there are simply repairs that are covered, and those that are not.
For instance, the repairs for a failure of the buttons on the back of the camera within the warranty period, on a camera that does not show any evidence of mishandling or misuse, would likely be covered. Repairs for a camera that is “bricked” or otherwise having issues directly related to using a third party firmware would not be covered.”
Canon doesn’t mention anything about EOS-1 bodies and third party firmware in the above story. However, I have been told Canon would take legal action against anyone publicly releasing unauthorized firmware for the EOS-1 bodies, so this is probably why we haven’t seen any development of such software.