A friend suggested we do a side project that does exactly this type of overlay to help with accessibility.
It is a very powerful pitch to be able to say “install our turbo 3000 power a11y plugin and your website will magically be wcag 3 compliant.” so why not give it a shot? The market is definitely there.
I spent a week or so reading up on wcag and doing some prototyping and came to the conclusion that this is a very hard (impossible?) thing to do right, and ultimately I felt like we would be doing people a disservice by even trying. We would absolve the website owner from having to do the right thing. In the end I backed out.
I would happily help clients make their website accessible. But this quick fix thing because you decided it wasn’t worth your time does not sit right with me.
If anything, an overlay creates new accessibility barriers. It’s why ad-blocker filter-lists for overlays are getting some popularity right now.