After a bunch of back and forth and many hours of trial and error, I've finally got a robust way to integrate the Clipsal/PDL Iconic dimmers integrated into Home Assistant via zigbee2mqtt.
I have submitted a PR that adds a zigbee-herdsman-converter that means the dimmers should be recognised automatically by zigbee2mqtt, allowing them to be controlled by Home Assistant like any other compatible device.
As it transpires, this was actually really easy once I worked out what to do,