Patient: Doctor, my elbow hurts every time I play tennis.
Doctor: Stop playing tennis.
I don't mean to make light of your concerns, but you are probably one in a million. Like all manufacturing, Canon quality assurance is geared toward satisfying up to the 99th percentile. You are not in that range. Honestly, I'll bet they truly believe what you're asking for qualifies as customization. They won't tell you that, but I think that's close to the reality of the situation.
They are not going to change their processes to accommodate you. I think you'd be well advised to accommodate your specific needs to Canon in this case. If you don't, in the end, they'll probably just send you a piece of black tape to cover your sensor!