Can damage be caused to a camera's imaging sensor (any size) simply by reversing, or stacking, lenses? Would stacking lenses cause a concentration of light to strike the sensor and perhaps damage some pixels? If damage can be caused by this configuration of lenses, what specifically would be the damage?
I can't imagine it. Rather the contrary, stacking lenses reduces the amount of light projected on the sensor. The larger reproduction ratio also means that what light does make it through to the sensor is being spread across a larger number of pixels, so less light is concentrated on any one pixel, though reducing the likelihood of damage (though, in fairness, if you had a bright enough light that it caused damage anyway, it might damage a much larger number of pixels).
It might still be possible to do damage by pointing at something like a welding arc for a long time, but if so, you'd be even more likely to get at least some damage if you didn't stack lenses.