My current solution is to use the hold function on one of my FC12 switches.The MLM (Master Layout), on layout 9, is like a central jumping-off place to other layouts. It might be possible to tell it to load with an external switch, but usually it's a two-switch combination and is always available on the FM9 or FX3 if I remember correctly. You wouldn't need an external switch to jump to it then.
On the FC12 attached to my FX3, it's the bottom right switch, hold it down, then press the top-right switch.
PS - see p. 11 in the Footswitch Functions Guide.
It sounds like you're familiar with the layout manager. The default FX3+FC12 layouts have a lot of real estate to work with, almost too much. I decided to default all my FC and FM9 layouts to use the top right switch for tap-tempo as a tap, and tuner as the hold function, then, if a particular layout gets too crowded, I'll use a per-preset override if I need to pull something else in and have something that doesn't apply to that particular layout.My current solution is to use the hold function on one of my FC12 switches.
It would be nice to just tap a switch to toggle between two layouts.
I am currently using an FC12 switch for tap tempo so I could also use that switch for layout toggle and use the external switch for tempo, then I lose the tempo read out though…