CAR 5.40 is actually pretty clear, its just that it doesn't support what have been long held views / operating mthods in Oz Aviation. However it is interpreted by various CASA FOIs and airlines, the fact remains that a Supervisory position is a company appointment and is not required for the logging of ICUS as per CAR 5.40.
The PIC of the aircraft must be an ATPL holder, however the PICUS must have a Command Endorsement, CPL (or ATPL) and have a valid MECIR. The reality is that the biggest prohibition to this is industrial (ie the point made re the REX EBA) and not regulatory.
I don't see a problem with FOs logging their sectors as ICUS, providing that they meet the requirements of CAR 5.40. What is the difference between this and the P1 / P2 system that seems to work so well in other parts of the world? If a C & T system is good enough, standards aren't a problem. I also don't think its neccessarily a big problem industrially. It will still take approx 2-3 years for a cadet to attain enough experience to meet the requirements for an ATPL and command. This means quicker commands for everyone else.
Cadet programs are never the whole solution to a problem for this reason.