Permit me, please, another thought problem:
A C-130 with a failed #4 starter lines up behind another C-130 for a "buddy start". The "mother ship" runs up a couple engines while standing on the brakes. 100% RPM, rated torque, zero airspeed: No thrust horsepower relative to this stationary ship.
But the recipient ship feels the propwash, and its #4 prop begins to windmill, finally permitting it to reach self-sustaining speed.
Obviously there is useful horsepower available in the propwash, even though it's not being used by the mother ship.
So my conclusion is: Horsepower is in the frame of reference of the beholder!