An interesting point JetA_OK. What's the definition of ICUS? Obviously there is only 250 hours required for HCAOC, whereas REX's operation requires 500. If candidates were able to simply log ICUS from the right seat during normal line ops (as hoped for by REX), why even have the requirement of the AOC in the first place? Within a little over 12 months everyone would have in excess of the min Multi requirement!
The problem for REX, and the Regulator, is there has to be some oversight (and accountability) to this sort of practice. If not, they would simply be doing it already. And I can tell you they are not. What requirements will define an ICUS flight as opposed to a non ICUS flight? What form will the supervision take? The proposal effectively attempts to circumvent the requirements of the AOC by proposing no requirements whatsoever! That's where the serious issue lies. I don't know how QANTAS do it JetA_OK, but In Command Under Supervision, means IN COMMAND UNDER SUPERVISION!
As far as the ATPL is concerned, you are right. Candidates do not even need Multi time for that, and the balance of the 100 hour requirement could probably be made up without too much trouble.