This is a lifesaver!
Do you have plans to allow (path) constraints to be merged, too?
Regardless, thank you so much. You've saved me hours of work as-is 🙂
Esoteric, I know you have other stuff to do before addressing these things in the editor itself, but I do run into this same thing from time to time, where I import another skeleton and try to reparent a bone from one skeleton to another.
I actually try this once every few months, forgetting it's not possible, because it seems like such a natural thing to do 🙂