Re: what's the most "efficient" way of doing this?

[Frédéric Bron]

> B.
>     B1. Put the RehearsalMark and MetronomeMark objects in the main global 
> variable (which also includes \time, \bar, etc.).
>     B2. Create a MarksOnly custom context, which only has the engravers 
> necessary to show RehearsalMark and MetronomeMarks, and let it "filter out" 
> everything else from the \global variable.
>     B3. Put the \global variable in all other contexts (for parts, etc.).

That's the best solution, at least the solution I used. You can have a
look at this score:
https://sites.google.com/a/polytechnique.org/bron/home/music/schumann---4th-symphony
first look in Schumann_Symphonie4.conducteur.pdf

I have three special text lines:
1. at top: text + bar numbers
2. in the middle: text + bar numbers
3. at the bottom: bar numbers only.

Does that correspond to what you want to do?

Frédéric