Hah yet again TK to the rescue!
I don't have a proof of concept yet but what you have sounds rather fantastic and more than I was thinking. For me, if I was going to use a separate effects box the biggest thing was envelope syncing. Not sure how to do that, but it would be nice to know that when I played something on my synth, the filter could follow it (ADSR in other words). I guess I could send note data out to both my SID and the MBCV and MBCV can use the note data to know when to trigger the envelopes?
Other than that, having the filter have it's on modulation things could give rise to some really interesting things I hadn't thought of until you mentioned it.
In terms of the control surface, I would love to have another use for my modular matrix boards I'm designing in addition to the SID and if there is going to be a sequencer in the next CV app, that might be cool to have that as an option for the control surface?