You can made a custom cable where 3 pins are used for the AES/EBU audio input and other two pins are used for the comms of your actual system.

I already tested it (the comm part), I cut an CAT6 cable and keep the Pin 1-Master Comms and Pin 2-Ground connected to the MC200. I checked and I was able to turn the speakers on, off and control the volume. If can custom made a cable that goes to you actual controller and connect it with your sound source in the RJ45 that goes to the speakers or you can create an RJ45 splitter, where in one side you only have the audio source connected and in the other only the comms.

I also tried RS232 with the M6, I was able to control it, but the M6 does not transmit the RS232 commands for the right speaker. This way I was only able to turn on/off/vol+/vol- in the master speaker, I don't know if this happens only with the RS232.

One advantage of the RS232 in the case of the M6 is that I can use it to control EBA, EBC, Loudness and others things (but it need to be done one speaker at a time). The MC200 doesn't offers be this kind of sound controls.

Your system is way more new and advanced than mine, but some of the logic might apply even in your system.

Cheers.