Some prosthetic limbs can be controlled by tensing muscles in your shoulders which are hooked up to electrodes. These muscles aren't redundant, but they aren't completely necessary to function as far as I know.
Here's what I've been wondering. Are there any places in the human body that are the opposite? That is to say, could there be any convenient places that could be stimulated somehow by a device, acting almost like another one of the senses, without it really interfering with everyday life?
I imagine that any of these places could be sorted into two rough types : one type that can gauge the strength of the stimulation, and another type that is accurate enough to only reliably detect "stimulated" or "not stimulated" (this would probably be more of a spectrum). If you can, please try and determine where your answer might fit on the spectrum. I'll look forward to seeing people's ideas!