Getting a little James bond here, but could you not design the reader so it fits in a band clamped around the power cable? Most users, even assuming they noticed it, would probably assume that it's the same sort of thing as the ferrite core on display cables. In my experience, most people seem to assume that is some sort of handle to pull out the cable. Even assuming they notice it, they will probably think it needs to be there.
These clamps could be fitted by (say) the cleaner in the morning, and taken away by the same cleaner a few days later. Design it right, and the device could be installed or removed in a few seconds, and a doubt anyone would question a cleaner hanging round a computer for a few seconds.