My pleasure good sir! This does seem to be a recurring problem.
Some of CAs models do assign to the weapon bones, apparently
without harm in many cases. I just remember that Caliban warned
us about weird animation effects occuring when trying to do things
with any of the weapon or shield bones. JMRC and I saw this when we
actually tried to put in animation data for one of the weapon bones
to do slingers and saw floating weapons, or weapons that started with
the unit and then got left behind when the unit moved. That was the
only reason I suggested trying the assignment to bone_Rhand.

This is only speculation, but extra bones in general work ok, but there's
some left over mechanism in the game that was never fully implemented
(this is the speculation part) that makes any of the weapon or shield
bones not work right if you try to animate them. Or in this case, if
you simply want them to follow their parent, bone_Rhand, which they
SHOULD do if they were regular bones.