It would give the ability to much more customise drops on specific Bows if you could track the Owner and the Bow that it was shot from.

A "Easy" (But Quick and probably Dirty way) to achive this would be to track the Tool the Player is Carrying when the Entity is hit.

This should already be partially possible using tool: PROJECTILE_ARROW@PLAYER;Zarius for example. Will consider options for tracking back to the bow (only possible way is to check the tool that the player is carrying) later.