search Wiki and forum for Marantz GSD code. If I remember right, Hari did GSD for Marantz as single zone device, and I did GSD code for Marantz as 2-zone amplifier (it's somewhere on older Mantis as contributed enhancement). I also did GSD for ESS speaker selector (posted on wiki). I guess your device is something like "Marantz + ESS".
I see that MC-66 is pretty close to ESS device. MCA-66 is more like addition of amplifier - like Marantz... What I did with ESS is to make template for generic audio zone (with source selection and volume controls) and then created parent device as DCE container (all DCE messages will arrive there) and as many zone children as there are in your device. Then you connect all your audio sources to separate child zones, while you do all DCE-GSD processing in parent device and send proper sequences over rs232 to actual device. I think this is pretty sound approach for all multizone/multisource devices...