I don't think this is the answer to your question but I'll leave this answer here in case searches with similar keywords come across it:
I wrote a script called dr2dr.py that is meant to allow selecting individual controls from other dynamic reconfigure servers and combining them into a single server. If server 1 had controls A, B, & C and server 2 had controls D, E, & F a new server 3 could be configured that has controls A & F for example. Behind the scenes changing one of those controls on the front-end server 3 would result in interaction with the appropriate back-end reconfigure servers 1 and 2.
I haven't used it in a while a didn't leave a good demo behind (only this), so if it is of interest I can see if it works, fix it if it doesn't, and make a simple example.
In your case hokuyo_node has a dynamic reconfigure server, so the above might be useful, but I don't see a cfg in dynamixel_motor, you would have to add that functionality. I didn't look at it closely but if there are other interfaces (like topics or services) that allow changing joint limits within dynamixel motor then you could write a dynamic reconfigure server for your own package that uses the alternate interface behind the scenes. You can also manually have your reconfigure server call other reconfigure servers without dr2dr, that's just for some limited use cases.
If dynamixel motor only can be configured once at launch time then you are back in the position of having to modify it to make live changes possible.
From your description it isn't clear if your main node needs a reconfigure server, it is going to configure other nodes based on processing it does and not real time user control?
