How about adding a "Solo boost" setting on the Main output screen? Maybe you could set a value to add to the current Main output level and have this assignable to a controller.
I know I could use a volume pedal, but I'd prefer a switch for a simple boost. I currently use a locked clean boost effect, but it would be great to not lose one of the effect slots for something so basic. And a boost is likely generic enough to be a global setting - not a per rig setting.
So for example, maybe you could set the boost for +3 (or any value) over the current master volume, then toggle it on/off with one of the assignable footswitches or maybe with one of the effect on/offs or in place of TAP (since it looks like Tap will be one of the switches on the KFC and it's probably too late to add a stand alone Boost switch).
What do you think?