Yeah - there is nothing in the 1st, 2nd or 3rd editions of the ADR's that I can find. There is nothing in the RTA vehicle inspection bullitens other than the numberplate needs to be "mounted". There is no definition of how it must be mounted. Even in the number plate inspection bulliten, it doesn't discuss how it is mounted, just that it is visible, distance from ground, talks about covers etc.

Beyond this, I know that there are certain cars which don't have any mounting points for the numberplate (especially on the front). For example, many modern Ferrari's where in Europe they have a "sticker" number plate on the bonnet, and in America don't need the front plate (in many states), so the dealers here just use double sided tape as well (at least in NSW).

Additionally, I've driven my car without number plates fitted with permission from the RTA. Albeit it was immediately after picking up new plates from them where I then drove from Castle Hill RTA to European Autotech at Mona Vale. So there are concessions you can even get from them.

I personally feel the dealer is just being difficult.
But if that's the way they want to play it. Make it their responsiblity to fit the Lakin brackets for you.