With regard to XPDR codes, how about a dedicating the last two/three digits to altitude(regional/fligh level?) e.g flying at 3500' squawk 7035, or an aircraft at 12500' will squawk 7125. I don't know too many people who fly at 50000, 60000 and 70000, so it shouldn't conflict with the emergency squawks.
What do I set if I fly at 8000' or 9000' ??
