HSV GTS was available (since VY I think) with Ohlins adjustable coil over suspension - an Australian produced car.
HOWEVER, I'm sure they had to make it comply with the relevant ADRs, and I think this is the issue rather than the fact it is a coil over or adjustable.
Any component that is fitted to a vehicle that has not been proven to comply with ADRs, they can force you to remove, or to prove it does comply. The latter is not a cheap exercise by any means.
I'm guessing the manufacturers of aftermarket Japanese performance products have no interest in gaining compliance with our design regulations, as the cars they make them for were never to be sold here. Sure, some of them may comply - but who's going to pay to prove it?