Use a regular X-speed cassette hub, one cog and a bunch of spacers. Any chainline you want, bob's your uncle.
+1. Probably also easier to find replacement freehub body, and replace cog when needed, than freewheel body with fixed cog or that allows cog replacement. Only question, if not using narrow chain and typical cassette cogs, but rather, wider single-speed cog and crankset, is if they make wider tooth cogs like that to fit on a standard pattern cassette body? But if using standard multispeed chain, not an issue.