[egor83]

  > If you were to produce something along the lines of that 
  > for open-source projects that need it, I wouldn't hesitate 
  > to pay a modest fee.

Try BountySource [1] maybe?

There you can post a proposal for some improvement to an OSS project (a design doc, in your case) and pledge a bounty for it.

If other people have the same problem, they might chip in further, making it more visible/profitable to solve.

----

[1] https://www.bountysource.com/

[Denzel]

Thanks for the link! First time I've seen that marketplace. I'll look into it.