Emergency furnace shut-off?

I assume you have an existing shutoff switch? I don’t know if there’s anything that makes it different from a regular lightswitch (except the red switch plate) - you could just replace it with a Z-wave toggle switch, or (if that makes you slightly uncomfortable, as it does me) you could install a motorized switch cover like a Switchmate (bluetooth) or EcoLink TLS-ZWAVE5 (Z-wave).