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They line up pretty well with the outer ring of a road double chainset. I have one of the older type (130mm), with a conventional threaded fixed side with provision for a left-threaded lock ring,and the chainline is 46.5mm -47mm, depending on the brand of cog. I think the newer type with the proprietary splined cog on the fixed side is much the same. You'd probably have more leeway on the free side with the use of spacers and freewheels of different widths.
Brian's lesser known brother?
There seem to be mixed reports on what the chainline of the White Industries ENO hub actually is, I think I'd measure it and then start looking at cranks.