Retargetting extra bones on a Humanoid rig?

I have a series of characters, each with a humanoid rig, but with a few extra bones for weapons and attachments. Each of them has the exact same hierarchy and naming convention, except for the base rig names which are different (e.g. John_Armature, James_Armature etc.). Can I seriously not retarget the animations on the extra bones unless I rename the rigs to the same things? It sounds like a huge limitation. Is there no workaround?

 

Hey. I'm not sure if you've solved this or not, but I was having this exact same issue.
You need to have the custom joints selected in the mask section of your animation on the inspector panel.
That ought to do it.