Boos actually used a pressure cooker, although they did cut their chicken up fresh each morning.
They also said they used wild peppers that grew on their land down in LA. They would process the whole plant also (leaves, stems, everything), not just the pods and dry and age it. They also had damiana in their spice mix. I think that accounted for the bulk of it, but I'm sure there were dashes of other things.
I've contemplated this for a while (and tried some other variations), and this is the direction I'm headed:
Indian chili powder
Brown sugar (v. small amount ground fine)
The proportions would need to be tweaked I'm sure, but I think this would yield something pretty close to what they served. The key is to take the chicken directly
out of the fryer and thoroughly coat with the spice mixture so it adheres well.
If you can still see the chicken you didn't put enough on.