\version "2.19.80"

#(define-markup-command (export-toc layout props file)(string?)
   (let* ((gauge-stencil (interpret-markup layout props ""))
          (x-ext (ly:stencil-extent gauge-stencil X))
          (y-ext (ly:stencil-extent gauge-stencil Y)))

     ; produce delayed stencil
     (ly:make-stencil
      `(delay-stencil-evaluation
        ,(delay (ly:stencil-expr ; this will be run, when all pages are calculated
                  (let ((table (ly:output-def-lookup layout 'label-page-table)))
                    (with-output-to-file file
                      (lambda ()
                        (display "Seite\tTitel\r\n--------------------------------------------\r\n")
                        (for-each (lambda (toc-item)
                                    (format #t "~A\t~A\r\n"
                                      (if (list? table) (assoc-get (car toc-item) table) "???")
                                      (markup->string (caddr toc-item))
                                      )) (toc-items))
                        ))
                    empty-stencil)
                  )))
      x-ext y-ext)
     ))
exportTocMarkup = #(define-scheme-function ()()
                     #{
                       \markup \export-toc #(format "~A.inhalt.txt" (ly:parser-output-name))
                     #})


\header {
  title = \markup { Hallo Welt! \exportTocMarkup }
}
\tocItem "Hallo Welt!"
\relative { c'' }