During your recent tests, has the OpenRB-150 unit that you were using had any components heat up to the touch or anything else out of the ordinary?
It seems more likely to me that the failure is with the OpenRB-150 than with all of your connected servos. You mentioned that you have access to a RaspberryPi, so you may be able to test your DYNAMIXEL communication using the UART on that according to the method used in this post.
Hopefully that can confirm if you are able to communicate with the servos themselves without the OpenRB.