So I had a flash of inspiration and amazingly it turned out to work great and is fairly easy to do. I'm not sure if this is the "correct way" to do this, but it worked for me.
You just need to change the device driver to "Virtual Thermostat" and hit save.
You'll then have the option to set supported modes, put them in as lower case and comma-separated, then click the button above the text. You'll then see the "Supported" fields appear as shown below. Hit save again to make sure the settings stick and then change the driver back to the Zen Driver.
You'll still see the unsupported options in the driver view drop-down menus, but if you check your dashboard thermostat tiles, the unsupported options will be gone. You will however still have the supported modes listed under "Current States".
