Wayne Gregory shaped the stays to the users back as I recall, as a custom option (the carbon was offered as an upgrade over aluminum). At least that was the state of things in the mid 80's when I got my first Gregory. Same currently with Kifaru I believe.
Absolutely nothing wrong with carbon fiber stays, just can't fathom not being able to custom fit the stuff... although likely not as critical with the full wraparound belt... which is another thing I can't believe some makers are going back to...
