I'm doing this from memory, so if I am saying something that is incorrect, then someone will correct me, of that I'm sure. I'm answering you because no one else is, so take my information for what it's worth.
Unless they are wire-guided, ALL torpedoes are essentially BOL. Yes, you can send them in a direction, and there may be a way to activate them further down their intended path, but I don't know how to do it (sub warfare is not my specialty).
Most wire-guided torpedoes in the game will remain "guided unless the firing sub exceeds 10 kts. At that point, the wire breaks and the torpedo immediately goes active. Using that logic, it would seem that the game is likely set up in a similar manner. If it's not guided, then it will start searching pretty soon after it is fired. Obviously, you can set the path for the BOL, and the sub will try to follow that path unless it finds a target, but the torpedo is sort of on its own once fired.
In answer to your question, I don't know how to set BOL and then activate later, nor do I know if it can even be done.
Hopefully this will help. If not, then hopefully someone with more sub knowledge will come along and answer your question properly for you.