Here's an an idea that extends the above procedure... once you have the shutter open, take the batteries out of the camera. The shutter will stay open; no power source to worry about. I don't have a D100 to try this out on, but it works with my F80 so I am thinking it may work on a D100 as well (probably most other Nikons too?).
When you're finished swabbing your CCD, replace the batteries and turn the camera back on. The LCD will flash "Err". Just release the shutter and the mirror will flip back up and "Err" will be cleared.
Remember, I don't have a D100... just a thought, not a recommendation per se.