If it's a single limb and the animation is vertex based, and not limb based then you won't be able to. I've not tried it but you might be able to use both vertex deformation animation with limb based on the same mesh, i.e. place the sword as a separate limb on your character and animate it so it's a separate limb, then you will be able to simply glue your collision box to the limb. But unless I'm mistaken there's no(native) way to get the bone structure/positions/rotations etc.
Quote: "Is there a way to either access the "sword" arm as a limb or use dbLinkLimb to attach a sword limb to the right hand of my model?"
I haven't used the animation features all that much in DBP/GDK but I'm pretty sure the skeleton used for vertex animation is inaccessible to the user and the object itself will in this case read as a single limb, so you won't be able to link any limbs as limbs aren't bones in the same sense.