Shortly after posting that, SmartThings announced that device handlers can now have child devices so CoRE is no longer necessary.
The “Multifunction” version of my GoControl DTH is meant to support everything so I left that one as-is and built the child device feature into my original GoControl Contact Sensor DTH.
You should install that DTH and the child DTH listed below it. Then switch your device to the new handler and change the “Main Contact Behavior” setting to “Internal Contact Only” and enable the “Create Device for External Sensor?” setting.
That should make a new device appear in your Things list that is synced with the External Sensor. You should open the settings screen and save so that all the default settings get applied and you might have to trigger the external sensor to make it sync the first time.