SMAC already used the concept of Scramble planes. When they are in a base (city) they takeoff any time an enemy unit try to attack your units inside city area (classic 21 squares).
I suppose you should have the same effect putting planes on alert inside airfields builded outside cities, for a similare range (I know planes have longer range, but they have limits to timely intercept any incoming enemy.
I'm not sure about any CAP mission (your planes are in flight, ready to react), because common planes rules in Civ don't manage well planes mid air, and I'm not sure putting too many tactic commands at work don't become a bore.
Civ 3 is not a wargame, IMO, so steer clear from that mermaids!