Hi,
Alternatively: Is there a way to load the necessary scheme code of the
functions in question by defining some \include statement for plain
scheme code files in the .ly file, ensuring they are loaded "after"
lilypond's files in /usr/share/lilypond/current/scm?
The way to do this is to write the plain scheme code files as lilypond files.
Put the functions in a .ly file.
Preface each function with # (to put the parser into scheme mode).
Then include the .ly file like you would any lilypond input file.
HTH,
Carl