Indeed, manufacturing is hardly a problem in the software world. =)

Maybe get it laser engraved into aluminium and mount it... somewhere? Grey code is essentially a more robust spiral pattern, so that's the same idea.

What if you put a roll sensor on the chain instead and infer crank angle from that? Chain slip is rare enough not to worry about it, right? You'd also have more space and could use off the shelf components.

I don't think measuring against gravity will be precise enough to be one in 7200 parts. So you'd need a second sensor for that in any case (and likely already have, don't you?).