I called them, and you're right. They only work on stuff they manufactured. No OEM or competitor's.
I also asked them if their aluminum A199 calipers, which are sold as a K-H replacement part for 1965-'72 disc equipped A-Bodies will fit K-H style replacement pads. He said they will. So you could run whatever pad material you want, and not have to use their proprietary semi-metallic only pads. Those calipers are expensive, but when I see rebuilt K-H calipers selling north of $400, brand new aluminum for a bit more isn't outrageous to me. I still wouldn't buy one of their kits, but if I were stuck for the calipers I'd consider them.
I asked if they would consider making replacement rotors for 4.5" bolt pattern. He said they do, but only for their kits, which require drum spindles and SSBC caliper adapters. He admitted they just machine down Mustang rotors to fit the drum spindles, but didn't seem interested in making them to fit OEM K-H brakes for A-Bodies. I told him it would make sense since they were offering replacement calipers. Whatever, maybe I planted a seed in his mind.