Paul,
Just thought I would include everyone to what we briefly spoke about earlier in the week, now that I have some more info.
To bring everyone up to speed with the conversation. I went to purchase some H&R spacers from AKS, when I said that they were for steel wheels, I was advised that the spacers are not meant for steels, only alloy wheels?
AKS did some digging with H&R who came back with the following reply:
We never tested the T6 on steel wheels for spacers and so we honestly cannot tell what
happens when you use the spacers on steel wheels.
The material should not be a big issue, it might be more that sometimes the tolerances of steel
wheels are different from aluminium wheels which can lead to the wheel not fitting
on the spacer or being too tight (which then damages the spacer / spacer-centering.
From reading this and to put a closure to that question, in my personal opinion I would be happy to take that as a low risk. There is no threat of failure, only that the wheels maybe tight on the spacer, all of which with a little care taken upon installing you can avoid if the tolerances are not too much out. Also the fact that because they simply have not been tested they have no evidence to say they are 100% sure they will be ok.
Looking at this sensibly though, if the spacer fits/feels right upon installing (hub is clean, no burrs, flat component faces and fully concentric when mounted) and the wheel bolts are the correct length for that spacer (as above comment by DX) the spacer is a solid piece of metal bolted between the wheel and the hub, once tight should be absolutely fine.
Arnie.