Yeah, if you really don't want to touch anything on the case, putting the switch behind the trapdoor or the sidecar door seems like a good idea. You can open the sidecar door without even having to lift the computer very much, and by that route, you can mount the switch to the RF shielding nearby -- just make sure you don't short anything.
Personally, though, I've always drilled the plastic and tucked a small switch into the back panel by the ports if I've needed to add one.
