This is totally brilliant, and works perfectly.
Here is an interesting technical documentation issue, which is that I had no idea this has anything to do with extra-spacing-width, and only hit on it after long searches of the web and the archives, then based on somebody's guess. Somehow there needs to be some _explanatory_ document that actually explains these concepts in lilypond, because in this case and others, it's non-obvious (to my dim consciousness). I'd seriously offer to write such a text, but I am exactly the wrong person to do so because I am the one that is baffled still in general by these internals.
I cant be the only one to want to write over the span bar. Could this be an LSR snippet?