I echo CM's response, 7000/C. If you've got it, use it.
Beagle - being at a unit that interrogates every 4 seconds, it is entirely possible to watch a transponder change through all 4096 codes (OK a bit of poetic licence). This can get extremely confusing when the Code Callsign Conversion system is trying to play catch-up. Before you know it you have a C152 out of Elstree indicating that he is BAW1 routing via Dublin.
A military ORCAM ssytem, now that's a good idea. The major stumbling block is the lack of networked communications. Come the replacement of MSSR (allegedly due 2006) we may be able to do something about this; don't hold your breath. In the meantime, get those fingers on your knobs!