Comentário oficial
There's no input MIDI mapping control for the auto-gain because it's not intended as a performance function. That, as you say, is the kind of thing you set before the gig. If you're live AT a gig, you would simply adjust the gains on the mixer channel to adjust it, or click over to the grid adjust panel and dial it over to where you need it.
I understand the ease of using a hardware knob to make this change, but I, for one, wouldn't want to have any accidental changes made because I hit the wrong knob while the track was loaded.