I'm going to play devil's advocate for a moment and quote an email I received from Drummond Motorsport when I enquired about their coilovers
I read this as "for optimum performance, you should lower your car by 20mm at most" which makes the 50mm amount seem pretty reasonable.
I still think it's a bad idea. Last year I went camping in Victoria and drove my R33 down packed to the bullshit with a friend... with a combination of this legislation, the additional weight inside the car, the compliance of my standard springs and the sag of my 14 year old standard springs could very well drop my car into defect territory!
Still, I think it would be difficult to enforce... the enforcer would have to have a book that covers ever single make, model and badge/trim level... that will be a MASSIVE book