I do want to be able to encompass dynamics and articulations along with note columns, so I've gone with your suggestion of a custom engraver . . . or rather customizing someone else's beautiful example :)
I adapted the regression test "scheme-text-spanner.ly
" (thank you to whoever provided this!), and I've got something which mostly works. There are some problems, though, and I'm not sure I'm approaching this in the way you've suggested.
First of all, is going after the stencil rather than the left and right text (as I did originally) the tactic you had in mind for both approaches?
The first snippet looks fine (both the dynamic and the articulation are encompassed), but the box won't expand to cover both the dynamic and the articulation in the second example. Also, the distance between the systems doesn't change. (Raising the last pitch a couple of octaves will cause an overlap.)
This is the part that is way beyond me. . . I'd be grateful if you could give me some pointers here, because I've hit a dead end as far as blind experimentation goes . . .