Maybe we need a Rochester Arduino group?
Combover, I was thinking something very similar. I want to get a "broken" mixer and wire all the pots up to the arduino like the one I made. That would give an extremely clean look, give you higher quality pots and might even be cheaper. Certainly cheaper than buying 13+ quality pots.
I'm not familiar with the workings of an analog mixer but I don't know if you could use it for midi signals and mix live audio. The live audio will probably make the arduino read funny values.