Why you cannot use bone as joint location? What is the reason? Only one that i can think of is that you want move joint position in game time. Editor can add/remove joints, maybe you can do same from C++ or even BP.
Anyway i think that after you connect joint actor to 2 bones. You technically end with 2 constraints. I did not test it, but if you move joint actor to location of “Specific point” you still have one constraint connected to “Hook joint” and another to center of your box.