I see your approach to it all, but I don't know if I agree with you Sir Ian; from my POV, the point of any martial art was guided by one underlying principle, regardless of which style you choose to study- to impose your will on your opponent and win. HEMA/WMA, as you say, is to figure out what was taught, what was most effective- but the sole purpose of any of these arts was to best your opponent. Nowadays, we have the luxury of taking up martial arts for any number of reasons- physical fitness, to learn discipline, holistic balance, and/or to defend one's self, or to study a lost art just for the sake of it or purely for sport as it is with most of us here- or any combination you care to come up with; still doesn't take away the original purpose of said art. Also, I might add that the sword was made for but one purpose only- to kill. Axes, maces, hammers, bows, arrows, daggers, knives, quarterstaves- all of these sorts of weapons had other uses, domestic and otherwise; not so with the sword. That people have used it for other than what it was intended changes nothing. And from a sporting perspective, it is always a competition, is it not?
As far as proving one's self goes...since Mr. Clements has given the bird to pretty much anyone outside of his circle, I don't see why someone similarly skilled in another style (or even the same, but from outside said circle) couldn't issue a challenge and have it accepted. Naturally, he need not answer it- that's his choice. However, and this is just my POV but if you're gonna talk the talk, you should also be able to (and willing) to walk the walk.