Would it be feasible to design a passive CVT gearbox that converted input rpm into an output of always N by splitting the input (such as is split in a helicopter transmission) then inverting the speed from one split-torque gear to merge with the input from the other (both inputs undergoing several transformations) for instance in the ring of a planetary gearset?
If I understand correctly what you are looking for (and provided that I translated it correctly), then a power-split transmission or power-distributed transmission should get pretty close to it.
It's a common transmission in tractors. Basically the rotation from the engine gets split into a mechanical part that ends in a planetary gearset and a hydraulic CVT that drives the ring gear of the planetary gearset. Via appropriate settings of the hydraulic CVT, you can control the RPM of the mechanical driveshaft continuously down to 0 RPM, that is, the ring gear turns against the sun gear and exactly cancels it out.
Your application sounds to me like looking for the reversed process, but maybe the concept is helpful.
I don't have a picture at hand right now, but can add one later.