Beyond testing that way, once you're using MF, there are no problems you cannot work around. Why send the actual CC that you want when you could send a different message (e.g., different channel, message + 30, or a sysex) and then alter it so that even if it echoes, it wouldn't be controlling your MIDI destination.
I'm frankly nott sure how CoreMIDI works in this regard, but my guess is that multiple recipients will all receive from the MD-initiated port.
Good luck investigating, it sounds like fun, honestly.