Any issues using the Hive without the hub? They sure make it sound required... Any features that don't work, etc? And HA can fully control the Hive thermostat via Zigbee?
Or hook it up to HA and send the on/off commands as you wish.
As for HA, there's really no dedicated service or add-on for Hive. But the generic thermostat service is good enough.
I combine the scheduled heating in Hive with "on-demand" through HA when needed.