I imagine that this would have to be done by the other units midi. You would send that CC number and apply it to the desired parameter, as long as the receiving devise is set up to react to that, you should be fine. When you have a midi in and out connected, each message would pass through. Unless that device was the required device you wanted to control.
Lets say you have a master keyboard and a bunch of rack units each on a different midi address, each has it's own CC values and address in which to communicate. If you wanted a control a unit at the end of the chain, the messages would just pass through the other units and get to the desired unit via it's address and CC.
What specifically do you want to do?
Cheers, Stuart
