This is the function as it ended up in my project files:
% Print table of contents entries to <basename>.toc
% Based on
%
http://lilypond.org/doc/v2.19/Documentation/usage-big-page#sharing-the-table-of-contents
% Pointed to by "Thomas Morley"
#(define (oly:create-toc-file layout pages)
(let*
((label-table (ly:output-def-lookup layout 'label-page-table))
(toc-items (toc-items)))
(if (not (null? label-table))
(let*
( ;; We need a page-offset so entries are offset to begin
with p.1
;; (for use in \includepdf)
(page-offset (- (ly:output-def-lookup layout
'first-page-number) 1))
(format-line
(lambda (toc-item)
(let*
((label (car toc-item))
(text (caddr toc-item))
(page (- (assoc-ref label-table label) page-offset)))
(format #f "~a,subsection,2,{~a},~a" page text label))))
(formatted-toc-items (map format-line toc-items))
(whole-string (string-join formatted-toc-items ",\n"))
(output-name (ly:parser-output-name))
(outfilename (format "~a.toc" output-name))
(outfile (open-output-file outfilename)))
(if (output-port? outfile)
(display whole-string outfile)
(ly:warning (_ "Unable to open output file ~a for the
TOC information") outfilename))
(close-output-port outfile)))))
page-offset calculates the offset of the first-page-number versus '1'
and subtracts that from the actual page number of the TOC entry. That
means when your score starts on page 16 a TOC item on the third page
of the score will still get a TOC entry with page 3, and \includepdf
will generate the appropriate TOC entry in the enclosing LaTeX
document.