Manufacturing a single cassette cog is not particularly challenging -- no more so than a chainring, which are made on small scales, and which I've made in my shop. Any problems would likely arise from not requiring a single cog, but rather a group of them all riveted to a holder, which obviously increases complexity. And, of course, finding someone who can and will manufacture such for a price that the user is willing to pay, and that might be the eventual roadblock.

Any required spacers are almost trivial to make, now. Just draw whatever you need and have it 3D printed. Will cost under a dollar, plus shipping.