I'm not sure if this is better or worse then my back-of-the-envelope approaches:
1) Modify grub to accept hotkeys on the boot menu, then make the switch simulate a keyboard that keeps sending keypress codes for that hotkey until it detects an operating system load.
2) Configure grub to chain to a flash drive as its first option, then have that drive chain to your preferred OS. The switch selects which flash drive to use.