After a few false starts,I believe I have now accomplished my goal.

The image above shows that the red key has been dropped, I own the blue and yellow keys, and the pink key is currently owned by another player.
I have also added an alarm/siren when one team owns all the keys.
The diff's, new key images and sounds are in a zip file here - http://www.badongo.com/file/9930637
It currently has a problem, when you change game modes eg from KH to CTF, the KH HUD stays.
The code uses two new cvars - kh_keys and kh_keys_status. They are both transmitted with the stuffcmd.
kh_keys contains information about the players own keys and is transmitted whenever a key changes state - a key is dropped, picked up or allocated.
kh_keys_status conatins information about the status of all the keys and is transmitted every second as well as when a key changes state. This allows the key status to appear within one second of joining a KH server.
I hope one of the game developers can take my initial code and turn it into a useful modification.