Kieren,
I'm not sure if this fits under your umbrella of having a single top level
file, but what about this?
%%%% song-FV.ly
definedVariables = data
\include song.ily
%%%% song-SV.ly
definedVariables = data
\include song.ily
%%%% song.ily
\include [.ily files containing the notes, edition-engraver tweaks, etc.]
\header {
[header parameters shared by both versions]
[differing header parameters defined in global variables]
}
\score {
[contexts built from variables defined in \include-d files]
}
%%%%
Optionally, you could use the output names and book blocks so that you can
compile the whole work from a FullVersion.ly and ShortVersion.ly which are
basically a series of includes of the applicable songs.
Hope some of these ideas help or at least spark others,
Sam
