Thanks for the link to the KRB site - very useful.  I was confused as I had not seen any of these UCB codes on Nairobi road signs yet, but maybe that is expecting too much, too fast!
My concern with these codes is that the rendering on osm.org generates a big box because the ref string is so long - which then makes the map ugly.  And ugly maps are not user friendly. 
What to do? Maybe use an abbreviated version.
Put the full string in in a dedicated krb-ref tag like this "krb_ref=UCB3 Nairobi R3" and a short user friendly version in the general purpose ref tag like this: "ref=UCB3"
Any ideas/thoughts?