Danjo
Master Black Belt
I think there are three types of cross ranking:
1) Honorary rank - as a thank you.
I think this was more appropriate in earlier times (when rank was tested and earned for more often) and when people really worked as complete martial artists and there were fewer masters around.
Example
Another master (say from Jiu-Jitsu), comes to my seminar that I'm giving on Advanced Kenpo Self-Defense Fighting Theories. We meet, talk, and he brings a boatload of students to the next seminar because he enjoys the striking part so much. I love it because he's bringing my more people and helping exposure. We talk, maybe train together and share ideas, and as a parting gift, we award each other honorary Black Belts in each other's arts in gratitude for service and dedication.
I'm good with this - as long as it is disclosed as such. One important thing here . . . it's not a promotion for either of us. We would both be advanced dans (5th+) and we just awarded each other a 1st Degree. Why? As a thank you. We might mention it, but neither would claim to teach the other art, and we both (as mentioned earlier) would simply respect it.
I don't really like this one. To me, if something is honorary, then it should only be in certain very narrow cases. I remember when they gave Sean Connery a black belt in Karate because of his movie You Only Live Twice, where James Bond was doing training in the movie and used a lot of Karateka in it. It was ceremonial and no one ever would be in danger of believing that Connery earned his black belt from them. To me, unless there is a big public ceremony hyping the honorary status of the belt awarded, then it shouldn't be done.
For the sort of thing that you mention above, I think a certificate of appreciation is sufficient.
2) Equivalency Cross-Ranking
There are plenty of really similar arts around. A lot of Japanese Karates, Hawaiian-based Kenpo/Kempos, Jiu-Jitsus, etc. You walk into my school, demonstrate techniques and I think, "You are a Shodan . . . you don't know my forms yet, but that's irrelevant. You're skill is what I would put at this level, therefore, welcome." You don't know my forms yet, but if that's not the main thrust of the school - then that's alright. I think about Judo ranking where your rank is a direct reflection on your competitive abilities. You might walk in off the street, beat a black belt and receive your Shodan. If that's the structure of your system, that's fine - it's earned.
This one is totally bogus IMO. If the forms aren't the same etc., then it's not the same art and there should be no cross-ranking.
3) Historical/Ancestor Cross-Ranking
Similar to the above, but if your art derived from arts X, Y and Z, and you master style A, then there is something to be said about saying you have a certain level of proficiency in arts X, Y and Z - maybe not the same, but some. I'm ok if an ancestor art recognizes you at a certain rank because of it.
Maybe in the old days, but now there should be plenty of legitimate black belts to train under in a given art so that you would not need to be cross-ranked by someone from a parent art. When Adriano Emperado gave Ed Parker an 8th degree, it was just a show of solidarity between Chow's first black belt (Emperado) and one of Chow's later black belts (Parker). Parker didn't really need the rank from Emperado, and was in fact heading up his own system by that time. But again, it was a public show of support, and there was no mistaking what it was about.