Interesting discussion here - and not something that has directly affected me in the past (never needed or used wheel spacers!)
One thought that comes to mind is for 'modified' cars here on the UK roads. What I mean is that some people make up all kinds of parts in their workshops/garages or at local engineering firms - what is the ruling for these?
Obviously wheel spacers is one area to look at, but what about everything else, including 'standard' type items such as manufacturerd wheel bolts, discs etc etc. Do they all have to be marked and approved in some way? Sounds too obvious, but what about some of the cheap and nasty aftermarket wheels floating around - are these legal for road use?