A few months back, I found this Tactical nuke made by Reuben201103. What immediately attracted my attention is the fact that the missile doesn't need a target lock to be fired - in fact, the first targets you'd probably hit are the missile destroyers obscured by the local terrain. Needless to say, I immediately started to reverse engineer the missile and LOOT DAT AUTO-TRAKKIN' ROKKIT TING!!! WAAAGGGHHH!!! -
...Oh sh*t my accent is slipping again!

Ok, now back to the topic of missiles - first thing first, what we need to understand is that we need the system to "think" we're firing another weapon that does not require a target lock - cannons, rocket(pod)s, bombs, torps, etc. To do this, we need to do three things: first, we need to install the aforementioned other weapon FIRST, and THEN the missile; second, we need to set the two weapons to the same name; and third, we need to set the activationGroup of the aforementioned other weapon to -1 so it is never actually fired. With all three steps complete, now we have our most rudimentary auto-lock missile.

Now with the basic set complete, now it's the time to talk about some important things that will come in handy when we actually build our auto-lock missiles: first, the aforementioned procedure must be repeated for every missile involved - or the auto-lock will fail; second, any settings that influence the locking characteristics like lockTime, maxTargetingAngle, maxRange, and minRange are completely meaningless - the only things concerning that matters are those affecting the missile's flight characteristics - its speed, the thrust of its motor, how much fuel it carries, how quickly can it turn, etc. Another thing that need to be taken account of is that the firingDelay setting is also meaningless now - all such missiles will be treated as if the firingDelay is set to zero. To counter this, we need to include -FireWeapons into the activationGroup so that the weapons fire semi-automatically.

With all the setup complete, we have ourselves a working auto-lock missile! Here are a few more things we need to mention before we use it, though: first, because the maxRange and minRange no longer hold any meaning, you need to be actually sure that your missile can physically reach the target before its fuel runs out and that it wouldn't blow yourself up along with the enemy when it gets there. Of course, as the missile no longer aggros the enemies when fired, you can also make them (rather part inefficient) dumb-fire rockets with a variable max speed and thrust. Guided bombs can also be made this way if you set either the maxForwardThrustForce (to make it behave like a glide bomb) or maxSpeed (to make it behave like a rather draggy bomb but to hide the smoke trails) to zero.

Oh, and another thing: only "smart" missiles like the Interceptor, the Inferno, and the cleaver can be used as the base of the missile, as there's no target lock in the first place! So they also make rather shoddy AtoA missiles when enemies have a lot of countermeasures (the most important aspect of the guardian missile is that it can keep that target lock indefinitely as long as you don't switch to another weapon so with proper xml-ing you can get some truly gamebreaking weapons). Granted, not requiring a target lock also means you can potentially hit things obscured by terrain, but given that most air enemies hate you guts to begin with (so forget about the aggro part), and it's a bit hard to be obscured by terrain when both you and your target are thirty thousand feet above the ground, the points about such weapons is rather moot...
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Addendum: apparently the only way to make auto-lock anti-aircraft missiles is by a cannon set to AirToAir, and then a fire-and-forget missile also set to AirToAir.