Well, after monkeying around with things I came up with splitting the music data files with includes. So something like \include "
sectiona.ly" \include "
sectionb.ly" for a single part. etc. So a single part would be multiple includes and then the includes could be included where needed in the vocal score. However, it turned out that while this was doable in certain cases it wasn't in, all for the reason that I needed to put voices into other lines and merge them in sometimes funky ways that were not receptive to such a method. So the upshot was I abandoned the idea, but it might have worked well for something simpler than what I happened to be doing. Anyway, I do not expect to have to make too many further edits to either the full or vocal score so there shouldn't be too much double maintenance in any event.