Is there any way to lock on to watchtowers?

While driving across the wasteland, it is not uncommon to run across a bandit watchtower:

These are not terribly dangerous while driving (their machine gun doesn't do an awful lot of damage, and they don't chase you, obviously) and there are certainly quite a few ways to get rid of them (ramming them, or stepping out of your vehicle and using a sniper rifle on the operator).

The thing is, I've never managed to get a lock on them to properly take them down with vehicle mounted weapons. Firing blindly at them will work, but some are placed in such a way as to make that nearly impossible. Is there some way to do this that I am missing, or is it impossible to get a lock on them?

No, the lock on in the vehicles are only for vehicle to vehicle combat. You will have to ram them, hit the with the rocket launcher or snipe the gunner.