IX_Legion, a modified version of your first suggestion is actually a great way to determine "missile lock." Back when I first tried to do rules for these in high school, i figured that the best way to do this would to take a turn for lock on, and on the next turn (and all subsequent turns until another target is acquired) the missile would ignore all movement penalties. But that still felt really dumb to me.
Your method allows for the missile not only to be fired on the same turn, but with a slight tweak, actually incorporates the minifig's skill, since there's no reason a hero with a guided missile launcher shouldn't be strictly awesome with it.
So the way I see it, locking missiles should simply allow for a minifig to roll skill before he makes the shot, and then decide whether or not he actually wants to fire. It still takes an action, but doesn't deal damage or cost a rocket. And that's it! If you make the skill roll, you get missile lock and hit. If you're trying to make a precision shot too, you can keep targeting until you make a high enough skill roll (after bonuses for aiming etc. are applied) to land a precision rocket strike through an air vent on the nuclear fallout bunker. Anything with some kind of ECM blocker could simply add points to the UR from such a weapon. I too stay away from CP, so it's up to someone else to determine costs for this weapon-modifier. I suppose this might also work in futuristic settings with homing bullets or something like that (like the flechette rounds mod in Deus Ex 3) that one could blind-fire around a corner.