you could read the MIDI (or OSC) values TotalMix sends out as part of its control surface support. It supports the Mackie protocol (and maybe more, I haven't looked too closely but I do use it with a Mackie-compatible surface).
TotalMix sends out the value of any changes, so that the surface can reflect them. For example, if you move a slider in TotalMix, a connected control surface needs to know too, so TotalMix sends out the values as they change. You may need to enable surface protocol support in the settings, but I don't think you need a surface actually connected to receive the values via MIDI.
Or you could use a software control surface emulation program, then it should all just work.