I have this incredible unique idea. Why not follow the official CASA NAS document when flying en route below 3000 agl.
Page 27 under VFR airmanship- choosing frequencies
Multicom 126.7 - a recommended frequency to monitor when flying 3000' AGL or below or nearest CTAF /MBZ if different.
Now I don't think anything could be more logical or straightforward.
Wouldn't this solve the CASA stated problem for enroute alerting of traffic that may fly through a circuit area- yes. It was already thought about but CASA forgot to read their own document- incompetent!