The last thought in post #13 answers that question quite nicely, maybe there are local targetting sources that can point to where the sub went.
Presumes an in-flight re-targetting capability which is secure and a very sophisticated battlespace C2.
Ahh but here comes the crunch. Is it not easier, quicker, cheaper and more efficient for the source tracking the target to drop a weapon itself?
Lets say you have a high value target, with an escorting helicopter. The helicopter picks up a hostile sub.
Drop a couple of torps immediately.
Phone up HQ, request a fancy missile. Hope they're not all tasked to other roles or VOR.
Wait 10mins+, providing a track on the target, hoping your Datalink doesn't get jammed, no equipment fails or you loose the lock.
Pray your Helicopter doesn't get hit by the incoming missile.
Then if you miss, start all over again.
I'm not saying you couldn't technically do the missile, but I'm asking if you should. Especially if it relies on assets already on scene, which is almost certainly able to carry weaponry itself.