Using them on jump pads has been considered and exploit, and can be banned for it. I'm pretty sure it has been mentioned in a post before Ah found me a post that counters this state. So putting them on jump pads is completely legal. Though your still a ****** for TK's(Well Team Suicides) you may end up causing http://forums.station.sony.com/ps2/index.php?threads/engineer-turret-on-landing-pad-exploit.79014/
They only lock on when it is being manned. When they are empty the missile won't lock on it. For all ground to ground missiles this is the case. (I bought all rockets to test them). Kind of sad, in that, a manned turret is considered a vehicle.
I can use it as cover for my tank AND when im off engi kill all the stupid engineers using their at the top off very hill/tower/tall-thingy Honestly, EVERY missed lock-on missile should re-lock and hit the nearest MANA.