OK,
For the record, and yes I only have a 56K connection, so it took me ages to trawl through the tripe on the vicroads site.
This may be open to interpretation, as I think your question Dave is fairly sneaky. (i like it!) BUT because it is not part of the wheel from factory, then a spacer it is...
goto:
http://www.vicroads.vic.gov.au/vrpdf/rdsafe/vsi%208.pdf
it says:
Spacers between the wheel and hub are not permitted unless provided by the vehicle manufacturer as original equipment.
(note this is for victoria only.... any other state, I am not sure -and remember 56K connection!?)
Now why do you think they say this? Might be cause there are alot of backyarders out there who reckon putting spacers on wheels then driving round with only a couple of threads on their wheel nuts is a good idea. Not to mention the increased forces acting on your suspension components for which they are not designed.
Why do you have big wheels on your car in the first place? Is it because you want the 50psi turboed + nos'ed +etc. engined car to stick out when she's stationary? Well that big motor makes you go faster round corners, and in a straight line, which places higher G forces on your components. Admittedly, some of the components are over designed, but within reason. If you are going to be stupid, then something is going to fail. Can you pick what is going to fail? If you cant, then you shouldn't be doing it. If you can, then upgrade that part to ensure it doesn't fail and the safety factor is increased.
This occurs when you visit your friendly and well informed (well some of them are...) automotive engineer, who says "Oh you are running 22" wheels on your 120Y" well you better re-design this, and that, and change this and change that. WHY DO THEY SAY THIS? cause most of them know (and some of them don't) that certain modifications put extra stresses on components. AND some of them might actually simulate the 1G breaking force before you or your mechanic are fitting the component to your car for validation, they might even do a few calculations to confirm their initial testing....
As for datsuns being cheap cars.....120why? Cmon man, some of us have spend $$$ on our datto's.
If you do get a defect, you gotta stuff around and put the old wheels back on, get the canary taken off, then put the phat wheels back on again.
Personally, I will get satisfaction for being pulled over and the copper says "wheels are too big mate" to which I curtiously reply "Oh sorry officer, your keen eyes do not mistake you, but may I retort; i happen to have an engineers certificate in the glove box, which states the wheels on my car are 100% legal."
Look, if it is an exhaust, then I agree. There is nothing like pure 110 dB of SR20DET hitting you as your following a turboed datto/silvia etc. flooring the daylights out of it. If you get pulled up on that, whoop de doo. Canary, big deal. Its not safety critical! You aren't going to kill anyone (or yourself) for having a loud exhaust... (well actually...you might if someone sticks their head in there, and......)
BUT, if you are running spacers and you manage to have an accident, you had better pull those wheels off before the coppers see you, cause:
a.) the insurance company will not cover you
b.) your butt will be in debt for all the damage you created when you crash into that vicroads building when one of your wheel bearings/wheel studs/ alloy wheel/suspension component fails.
c.) you will be held liable for your own stupidity.
PLUS... Do you know how easy it is to ask any of these well learned individuals in this forum what wheels fit and what don't? What sort of offset to use, and what doesn't work? If they can't help you directly, then they sure as hell will point you in the right direction.
SECONDLY, it's not that hard to find the correct wheels for your car. CMON, its a datsun, many people have modified these things within an inch of their life!
Hope all this helps.