I did read it, and in entirety, but the problems I posed I still hold to be problems. I do not think "tethering" the muscles to be anatomically viable. I am quite aware of separate muscle groups react to one another, but that does not make them able to control one another. In spite of this, this is still a fantasy server so really anything can happen. And as for the "cons" which you mention, I did notice how there is the dry bone clause, but the clause which jumped out at me is the one specifying how it cannot be stronger than a normal limb. To me, this seems very likely that people would be roleplaying it in a manner in which it would be equal to a normal limb. While this is of course speculative, at the very least that first clause is unnecessary if you are to later specify that it is as weak as dried bone. I would simply not like to see people incorrectly RPing skeletal arms.
In addition to this, would the arm be able to wear armor? Could you coat the bone in metal? What are these arms really capable of due to their structural weakness?
EDIT:
To further this, I think it has serious potential for powergaming. I can see someone reading a factlet like "weight for weight bone is five times stronger than steel", and misunderstanding it as their dry, brittle, and lightweight arm can stop a steel sword.