\version "2.17.2"

#(define (volta-bracket::my-print grob)
  (let* ((bar-array (ly:grob-object grob 'bars))
         (bar-array-length (ly:grob-array-length bar-array)))

        (display "\n\n\nGrob array structure:")
        (for-each (lambda (g)
                          (for-each display (list "\nBar-array entry No. " g ": "
                                                  (ly:grob-array-ref bar-array g))))
                  (iota bar-array-length))

  (ly:volta-bracket-interface::print grob)))

\layout {
  \context {
    \Score
    \override VoltaBracket #'stencil = #volta-bracket::my-print
  }
}

musOne = {
  \repeat volta 2 {
     c4 c c c | c c c c
  }
  \alternative {
    { c4 c c c }
    { c2 c }
  }
}

\score {
  <<
  \new Staff {
    \new Voice { \musOne }
  }
  \new Staff {
    \new Voice { \musOne }
  }
  \new Staff {
    \new Voice { \musOne }
  }
  \new Staff {
    \new Voice { \musOne }
  }
>>
}

musTwo = {
  \time 1/4
  \repeat volta 2 {
     c4 | c
  }
  \alternative {
    { c4 | c | c  | c | c  | c  | c }
    { c4 | c | c  | c | c  | c  | c }
  }
}

\score {
  \new Staff {
    \new Voice { \musTwo }
  }
}