So, getting voltage fluctuations from my PSU will work, but if I were to upgrade to a higher efficiency device, I would no longer receive any data? I suppose that makes sense. Would it be more practical, then, to pull RPM readouts from my case fans?
No, because they aren't randomly spinning, they spin upto a certain RPM when needed and stay there till they are told to spin back down, or stay at an almost steady RPM through-out...