Looking at the map, if the Heathrow concept were adopted at Williamtown, the Inner Area which protects the RWY centrelines would extend well over the water and not help coastal aircraft.
The following NATS restrictions also apply:
4. Access to the airspace immediately around Heathrow Airport – referred to as the ‘Inner Area’ – will be restricted via PPR (Prior Permission Required) by telephone. Unless you really need to fly inside the ‘Inner Area’, you should plan to route around it.
5. The London CTR will be a Transponder Mandatory Zone (TMZ) with Mode S required.
It appears that if Williamtown did implement similar procedures, an inner area would be more restrictive than the current Class C zone and both the coastal and overhead routes would be unavailable. With the nature of Williamtown operations, telephone approval probably wouldn't work either. Then there is the cost of a Mode S transponder.