\version "2.19.15"

barLineAtSystemBeginning =
#(lambda (context)
   (let ((text #f))
     (make-engraver
      (acknowledgers ((system-start-text-interface engraver grob source-engraver)
                      (set! text grob)))
      ((process-music trans)
       (if (ly:grob? text)
           (let ((bar-number (ly:context-property context 'currentBarNumber)))
             (set! (ly:grob-property text 'text) 
                   (markup (number->string bar-number)))))))))

#(define (use-my-text text-list) 
   (lambda (grob)
         (let* ((orig (ly:grob-original grob))
                (siblings (if (ly:grob? orig)
                              (ly:spanner-broken-into orig)
                              '()))
                (text-list (cons "" text-list)))
           (let loop ((sibs siblings) (text text-list))
             (if (or (null? sibs) (null? text))
                 '()
                 (begin
                  (ly:grob-set-property! (car sibs) 'text (car text))
                  (loop (cdr sibs) (cdr text))))))))


music = \repeat unfold 20 {
  c4 d e f
}

%%%%% Engraver %%%%%
\score {
  \new PianoStaff \with { instrumentName = "" } <<
    \new Staff {
      \relative c'' {
        \music
      }
    }
    \new Staff {
      \clef bass
      \music
    }
  >>
  \layout {
    \context {
      \Score
      \consists #barLineAtSystemBeginning
      \remove "Bar_number_engraver"
    }
  }
}

%%%%%%%% WORKS BUT CAN'T BE AUTOMATED

\score {
  \new PianoStaff \with { instrumentName = "" } <<
    \new Staff {
      \relative c'' {
        \music
      }
    }
    \new Staff {
      \clef bass
      \music
    }
  >>
  \layout {
    \context {
      \Score
      \override InstrumentName.after-line-breaking = #(use-my-text '("7" "14"))
      \remove "Bar_number_engraver"
    }
  }
}