I am going off of my experience with playing planetary annihilation, but, moving celestial objects is just a matter of setting up the massive engines required to move it. I think FTL for these is sheer crazyness, if not impossible. But, attaching ship engines onto an asteroid and just programming it to fly on a set course doesn't seem out of the picture. It would look to be mainly a single system weapon, on able to attack targets within the same gravity well of the local sun. But, yes, defenses could be set up, but, even if you did manage to break up the comet/asteroid it would still rain down smaller fragments. Just a thought.