Interesting. Yes, as Fred noted in 2016, B:CP does not define ‘controlled variable’, it defines ‘controlled quantity’, but as Rick has often emphasized perceptions at all but the very lowest level correspond to a set of variables {v1 … vn} which in principle are amenable to measurement, disturbance, and experimental concealment (though often not so amenable in practice). Complicating this, at higher levels input to the PIF for the controlled perception may include signals from memory that are not presently derived from environmental input.
This is a case of the Test with naturally-occurring disturbances, inherently more difficult to control than laboratory conditions. Typical of naturalistic ‘real-world’ investigations.
That part of the Test would seem to require tracing all the way to intensities of {v1 … vn}. Maybe all that is needed is a sufficient but not exhaustive demonstration, sufficient to assure that the higher-level p does not depend upon anything imperceptible. Could easily get kind of murky. Perceptible (has input functions) but unperceived (not in current input of subject, whether or not in current perceptual input of the observer) seems more difficult.