BA used to use this technique on the TriStar. Early models had RB211-22B engines, later ones had the more powerful RB211-524B4s. The more powerful engines created a VMCG limit on short runways, effectively the aircraft had to accelerate to a higher V1 than it would otherwise use to get over VMCG.
By reducing the EPR to match the 22B full thrust condition they were able to use the lower VMCG figures that matched the less powerful engines and get off in a shorter distance for the weight. The only extra consideration was that you had to swear a great oath in the take-off brief that, if an engine failed after V1, you would not increase the thrust and invalidate the calculation.