@briney I honestly thought there'd be some good guidelines out there, but I'm coming up with nothing useful from ACS (American Chemical Society) or RSC (Royal Society of Chemistry).
Julia Winter's company (Alchemie) is working on some sort of algorithmic alt-text generation. You can read some of the basic ideas here: https://pubs.acs.org/jceda8/article/101/7/2880/156131/The-Lewis-Structure-Explorer-Accessible-by-Design and more https://www.linkedin.com/pulse/accessibility-crisis-organic-chemistry-what-instructors-really-mc47c/ and a commercial product: https://www.alchem.ie/piph
ACS has a committee that I would have thought would have guidance (https://www.acs.org/about/governance/committees/chemists-with-disabilities.html) but all I could find was "Complex graphics and some other sections of online material may still require the use of a human reader or sufficiently written alt-text to be fully accessible to individuals with visual disabilities. This is due in part to the fact
that no standard requirements for figure creation and alt-text to describe figures are in place for most online publications.”
Hey - if you find something good out there, please share! 