one option for using an acoustic guitar in an emergency would be to have an FX Loop locked on all Rigs/Performances. The Send would go to the PA but there would be no Return. In this scenario you would be able to switch guitars and step on the Switch to enable the loop to go to PA. You could also try the Parallel Path option.
I sometimes set up my Piezo equipped Guitars with two cables. One goes to the Fron Input on the KPA. The other (Piezo out) goes to a return on the FX Loop block. This lets me have Electric throught the Amp with all FX, Piezo through full signal path (usually with some acoustic rigs loaded in the Amp block), or combined with electric going through full chain from Stomp C onward and Piezo going through one effect then on to PA using parallel path.
However, I tend to reserve Slot 5 in all performances for an acoustic rig. I have 4 rigs for electric and 1 for acoustic.