You're barking up the wrong tree here. The solution isn't in the modular...it'll be via the Fireface. Since that thing can do piles of channels simultaneously, what you REALLY need here is a DC-coupled expander of some cheap sort. Connect it to the ADAT Lightpipe connection (you don't need high sample rates for CV/gate/trig work), and that's that! Just make sure that you get proper DC-coupled hardware for that, and you're golden. With that PLUS the Fireface, you'll have ample audio I/O on that device itself, and all of your control signaling will be split from the audio I/O, which makes things make more sense.