(Plus, I'm pretty sure that Canon Service can work on things inside the camera without messing up the imaging sensor, I mean, things do go wrong in there, shutter failure, etc., and Canon can fix them, right?)
As much as I loved using CPS repair to fix a "bump" on my 70-200 f/2.8 IS USM II, I did notice a few more dust specs inside the lens (when looking through it unmounted) after they opened it up. Not enough to ever care, but still, not fully dust free. It happens.
As for the issue - yeah, its a non-issue. I dont shoot in dark closets and only maybe if I am shooting the Milky Way at 12,000 ft 100 miles from any city would I ever have to compensate, and I usually AEB anyway. Exposure Meter only folks!
Tape. You should see the stuff I work with
I am sure somewhere there is a new inventory item on the Canon Repair Supply System -
(1) Roll 20' of Light Absorbing Adhesive Blocker - $100.