I had 8mm H&R hubcentrics on the front of a mk4. They exist(ed).
BUT.....
Damian makes a good point. The rear hub 'protrusion' is often deeper than the front. Basically the inner bore of a hubcentric spacer can only be of a certain depth in order to allow the spacer to have a hubcentric lip.
In layman's terms, an 8mm or 10mm hubcentric spacer might not fit far enough onto the hub lip/protrusion. The spacer won't reach the disc.
On the mk4 at least - to confuse the situation further - some mk4s had deeper rear hub lips than others. So some people got away with 10/12mm spacers, while some people could only fit a minimum thickness of 15mm, and lots of people got the hump.
Some success was had with an angle grinder on the rear hub lip, but I'm not advising that...
I've probably described that terribly, I'll do a diagram to clarify if needed!