"How would you characterize the system with regard to ranking? Is there any kind of established, solid curriculum that is used to bestow rank, or is a more nebulous judgement regarding a student's ability to react effectively to a threat?"
First I would say that there is no Kosho Ryu Kempo "System" of Martial Arts. The ideas of Kosho are taught by methods that are completely up to the individual teachers. Since most (if not all, I'm not sure) Kosho Ryu teachers today (at least in Hanshi Juchnik's group) all have prior experience in other arts, I have seen many use their old system's curriculums with some changes. (This is what I have done)
To be honest there is no set rank standard, that I can see, in the Kosho group I belong to. Most of the black belts I have had contact with are extremely talented - but each in their own way. But if you walked into 5 Kosho schools you will most likely find 5 different curriculums, and 5 different sets of rank promotion standards. This makes it challenging for students wishing to transfer from one Kosho school to another.
The great thing about the way I, and others, teach Kosho is that we can show practitioners from all arts how to add in the Kosho Ryu ideas to what they are already teaching. So for me I took my Shaolin Kempo curriculum and "Kosho-ized" it. I have had great success with this and my students who started off in Shaolin Kempo didn't have much transition problems when I changed. I've also been able to teach seminars at other Shaolin Kempo schools and highlight the Kosho ideas using techniques they are familiar with.
In closing I will say that the way Kosho is taught does not fit everyone's ideal way of teaching students. However, I like the freedom (at a black belt level) to explore the ideas taught more openly. I know some black belts in other styles that would never change a technique to make it better for them. Their teacher would not allow it.
Just my thoughts.
With respect,
John Evans