Yes. If you modify the attachment direction, note that MeshAttachment and SkinnedMeshAttachment are stateless and the same instances are used if you have multiple instances of the skeleton. Otherwise you can do it like Animation.FfdTimeline: set the Slot vertices to the attachment vertices, then modify the Slot vertices as needed. This way the state for your skeleton instance is stored in the skeleton (in the Slot) and doesn't affect other skeleton instances.
However, maybe a better solution is to use skinning. Just weight the bow string vertices to the hand and when the hand moves, the vertices will too. You could even have a bone dedicated to moving the vertices around if you needed, just because it is easier to move a bone than vertices. Skinning makes programmatic mesh manipulation easy because then you just need to worry about bones.
