\version "2.11.60"

#(define (bis-bar-number BarNumber)
  "Append bis to the bar number."
(let* (
      (fontMetr (ly:grob-default-font BarNumber))
      ; Set barNumber to the current value
      (barNumber (ly:grob-property BarNumber 'text))
      ; Append " bis" to it
      (bisBarNumber (string-append barNumber " bis"))
      (width (ly:text-dimension fontMetr bisBarNumber))
      )
  ; Store the new value back
  (ly:grob-set-property! BarNumber 'text bisBarNumber)
  ; and print it
  (ly:warning "Font is metric: ~a" (ly:font-metric? fontMetr))
  (ly:warning "text-width for bar number ~a is ~a with font ~a" bisBarNumber width fontMetr)
  (ly:text-interface::print BarNumber)))

music = \relative
{
  \override Score.BarNumber #'self-alignment-X = #center
  \override Score.BarNumber #'break-visibility = #all-visible
  \repeat unfold 4 { a1 }
  \override Score.BarNumber #'stencil = #bis-bar-number
  \repeat unfold 4 { a1 }
  \revert Score.BarNumber #'stencil
  \repeat unfold 4 { a1 }
}

\score { \music }