Programable mixer controls

I'm using this as backing music in a trio setting... one guitarist and two or three singing. I would like to be able to control the volume of the three individual instruments by inserting commands in the chart. That way I can bump the bass up in parts, drop the piano volume when some is singing their section, build to a crescendo or fade out the song. Lots of potential here for this feature.

Ideally, it would be wonderful to be able to switch instruments using commands, too. But I can wait for that one. :)