Controlling Totalmix from RME (or MIDI channels)

Has anyone tried controlling volume levels in Totalmix (the software included with RME audio interfaces) using BetterTouchTool?

I’d like to set up one shortcut to control the volume of my speakers and another one for the headphones.

Another idea I had was to adjust the MIDI levels instead, since RME exposes multiple MIDI channels.

Is either approach possible with BTT?