Skin codes were once used but were discontinued prior to TAAATS.
Reason is that there is not enough codes available for even half the Oz register to use. Many codes are allocated or reserved for special and military use and with common radar boundaries to our North codes must be allocated/reserved on a shared basis with those facilities where coverage may overlap.
But having said that, it should be no great problem to obtain a code for specific flights, especially if likely to be within radar range. Certainly if the ATC facility has the capability, then allocating a bin code on request would not be a great drama and certainly would provide a safety benefit at minimal or no cost.
"No known traffic"
