Man, I must have slept through this. "this is already supported in
2.19" is misleading if it's actually only supported _outside_ of 2.19,
namely by chancing upon people in the know in the mailing lists.
The problem with that kind of support is that it's unreliable. Stuff
might get reimplemented because people cannot find what they are looking
for, and the old code might get removed as bit rot at any point of time.
To actually move it to "supported" state inside of LilyPond, there need
to be regression tests (which also stop bit rot), user-level
documentation and a Changes entry. That gives a new feature a
reasonable chance of getting tested and consolidated in order to be
useful for more than a single application (often by a single person) in
its region of interest.
Do you feel up to getting that kind of support into LilyPond?