Happy to help out! I would like to try this Sum.Mation box sadly I have not tried it yet, so its hard to give any real world advice here. Though looking through the manual, I think its a slamming concept! If you can patch your DA converters into the Sum.Mation, with its Fader Automation system, connected via USB to the host computer [along with the USB - GUI application of the Great River for snap shot recall] and patch it's outputs directly into channels of the MM20. Seems pretty cool to me!
You will be able to automate with the Sum.Mation Interface Plugs, leaving your DAW Faders [physical DA outputs] at unity so you can patch your compressors/EQ or whatever you want on the output of your DA converters, but you would then program automation on the Sum.Mation Unit, so as to trim levels AFTER outboard gear and before they hit the MM20. The MM20 has a WIDE sweet spot for mixing. Super Solid Sound, with great width and thick quality that is just awesome. It really sounds like a console to me. Maybe better than most! It has +29 headroom everywhere.
The First four channels of the MM20 are designed to bounce between Tracking and Mixing, so you might just have this gear on patch points, which you'll really want with the MM20, since its a center piece mixer with 7 DSUB's. It pretty much takes up an entire 48 point bay with all its connections. A bit more cabling will be required, but it will easily be worth while to have setup this way. I think the Great River is a very tactile Mini Console with huge, meaty flavor.
I suppose Dan would be the guy to ask about any future developments on that, but right now, the USB Driver for the MM20 is handled by a GUI app that allows snap shot recall for all storable parameters on channels 5-20. You save the presets on your computer for easy swift recall
The webpage mentions it, so I will definitely keep a close eye on it Adam! But I must say, even apart from that, the Mixmaster already has lots of great "console like" features and it just seems very solid and well thought out.
"Mix settings can be stored and recalled via the USB port and a GUI compatible with most digital recording programs is in development to make the MixMaster 20 an automated analog mixer."
Untill that comes available, the Summation seems a great solution to do post outboard automation. (to answer the topic!) While you're at it add some Bettermaker EQ's so you can automate post compression analog equalization.