My personal policy is if someone has put in the years of training to become a black belt, then I respect and acknowledge it.
I do not believe in "cross ranking", but they are welcome to wear their black belt along with the patch and uniform of the system they hold a blackbelt in.
Once they learn the Kajukenbo requirements they are welcome to test for a blackbelt in Kajukenbo. Then they can wear a Kajukenbo uniform, patch, with a black belt.
This has 3 advantages.
Number 1, you don't comprimize you ethics by cross ranking.
Number 2, you make the black belt feel respected in your school.
Number 3, visitor's and color belt students will be impressed that a black belt from another school/system is interested in learning from you.
If you cross rank someone, it will backfire on you. Because students and visitors will see a black belt in your school that dosent know your system. Makes the quality of your instruction and promotions look kinda low.