floating disc wear

Anyone know at what point the wear between the outer disc and the disc carrier becomes unacceptable. I have one of the Kenny Dreer Nortons with the custom made brakes by Kosman Engineering who I don't think exist anymore. On the rear brake there is about 1 mm slop in the outer disc both up and down and forwards and back due to the alloy carrier becoming worn. I was thinking of converting it to a one piece disc, but because it was a special I can't find anything with the correct OD and ID. The front brake which gets more use is still in good condition so I don't know why the back is worn. I'm tempted to put it all back together and just ride it but worried about a possible life threatening seizure. I don't expect this kind of wear when the bike hasn't even done 5,000 miles.

If you check out modern sports bikes, you might be surprised to see how common this kind of wear is. It's difficult to know without seeing, but from your description, I'd suggest it is fine to use as is.

Premature wear of the alloy could perhaps have been avoided if the alloy centre had been hard anodised?

There are companies out there who will make custom discs to drawing or pattern. I'll be doing that at some point to replace my RGM disc which is not as good as I'd first thought. This is the company that I'll probably use, click on 'products' and you'll see they do solid, semi floating, full floating and full floating with 'dampers':

http://www.isrbrakes.se/products/brakediscs/custom/