Ed, your description of Greenwell hackle is spot on. (Now I am about to make a spelling mistake, but its OK no one knows the right spelling) In A Dictionary of Trout Flies Courtney Williams says he has a letter in Cannon Greenwell's own hand that says his fly should be tied with a Coch-y-Bondduh hackle. Which is a dark red with black centre list and black tips. Not an easy hackle to find. It is considered "sub standard" by some breeders.
Cheers,
A.