The requirement that the ADSB transmitted position be the same as the position used by the onboard navigation system.
I have seen someone working on a discrete C146a GPS box simply to feed mode s transponders. I forget who, but it was one of the TSO manufacturers - the likes of Trig / Aspen / Sandel or maybe Becker. The idea is to get a cheaper way to feed the mode s transponder than upgrading the GPS.
So --- I don't know.
But I can see this being another 406 Mz transponder or Part 61 type implementation stuff up and at the 11th hour CASA will realise they have done something unworkable, back down and blame it on industry not being ready despite x years notice.
As a corollary, a friend noted tat a new car is pretty much always launched on time (or within a week or two) after what may have been a 5 year development. And, this is the industry that is fat and inefficient that we don't want. When was the last time we saw a government project delivered fully functional on time?