I dunno. Switzerland didn't really have a reason to take sides. It'd join in if it was invaded, but not much else. And there's really not a reason to invade Switzerland; very good defensive positions, elite troops, and pretty worthless land.
Have Austria or France invade Switzerland during that one catholic rebellion (forgot the name of it). The ending treaty gets some sort of annexation if by Austria or a defense contract if by France. Completely implausible but its the only thing i can think of.