The goal would be to have it in LilyPond, of course. My reason for
not putting this forward (now that I've found a way past the problem
mentioned in the quote) is that it's a stub, a sketch, more suited to
"unstable" releases than the impending 2.20 release. I should
probably hold off until there's a clear separation between material
destined for 2.20 and ongoing development. (I don't know anything
about a timetable.)
I alluded to a revised patch above. I could post it so the code would
be available, though you would of course need to build LilyPond to use
it. (In my experience, it's no big deal to set up and use LilyDev.)
-David