I must always agree with and acknowledge those who agree with me. Can not resist the temptation to whole heartedly concur. No yet anyway...But to address your question
Not quite sure what you mean regarding Stevens-Costello by, "If you were to pull this off it would first require the latent ability to blow forward jaw. This characteristic occurs only in a minority of brass players." Are you saying that the majority of people aren't able to protrude their lower jaw, because that's all he's suggesting.
No that wasn't my idea though I can easily see how one would make the inference. In fact I don't even know what the percentage of people is who can protrude their lower jaw is. My best guess is that most people can do this and yet would still fail at Stevens-Costello.
What makes the Stevens System so woefully inadequate for the great majority of trumpet players is that the chop setting requires an extremely supple upper lip texture. An almost "weak" looking lip which paradoxically allows the easiest production of unlimited upper range register. Most trumpet players do not have this kind of supple flesh where it is needed in order to pull off Stevens-Costello.
What is my proof that this i so? Its a theory but never the less is almost definitely the cause.
Essentially what happens in the protruded jaw (or "forward jaw") trumpet player is that the air passes through the upper lip at an area more near the outside of his lips. Or at a point deeper within the mouthpiece. Conversely in the receded jaw trumpet player the air passes over the upper lip more inside his mouth and LESS inside the mouthpiece.
Typically the inner gum membrane inside the upper lip of the receded jaw trumpet player get pushed to vibrate by the air.The tone from this sort of chop setting is usually excellent but the production of range far more limited. Although Reinhardt noticed that these kinds of players could "get a huge volume of tone" by playing this way...