\version "2.18.2"

startBarre = 
#(define-event-function (parser location fret-nbr) (markup?)
  (let* ((mrkp (markup #:upright #:concat (#:lower 0.08 fret-nbr #:hspace 0.3))))
    (define (width grob text-string)
      (let* ((layout (ly:grob-layout grob))
             (props (ly:grob-alist-chain 
                       grob 
                       (ly:output-def-lookup layout 'text-font-defaults))))
      (interval-length 
         (ly:stencil-extent 
           (interpret-markup layout props (markup text-string)) 
           X))))
    #{  
      \tweak TextSpanner.after-line-breaking 
        #(lambda (grob)
          (let* ((mrkp-width (width grob mrkp))
                 (line-thickness (ly:staff-symbol-line-thickness grob)))
           (ly:grob-set-nested-property! 
             grob 
             '(bound-details left padding) 
             (+ (/ mrkp-width -4) (* line-thickness 2)))))
      \tweak TextSpanner.font-size -2
      \tweak TextSpanner.style #'line
      \tweak TextSpanner.bound-details.left.text #mrkp
      \tweak TextSpanner.bound-details.left.padding 0.25
      \tweak TextSpanner.bound-details.left.attach-dir -1
      \tweak TextSpanner.bound-details.left-broken.text ##f
      \tweak TextSpanner.bound-details.left-broken.attach-dir -1
      %% adjust the numeric values to fit your needs:
      \tweak TextSpanner.bound-details.left-broken.padding 1.5
      \tweak TextSpanner.bound-details.right-broken.padding 0
      \tweak TextSpanner.bound-details.right.padding 0.25
      \tweak TextSpanner.bound-details.right.attach-dir 2
      \tweak TextSpanner.bound-details.right-broken.text ##f
      \tweak TextSpanner.bound-details.right.text
        \markup
        \with-dimensions #'(0 . 0) #'(-.3 . 0) 
        \draw-line #'(0 . -1)
      \startTextSpan 
    #}))

stopBarre = \stopTextSpan

%% Syntax: note \startBarre "text" notes \stopBarre (text = any fret number)
%% Example:

{ 
  \clef "G_8" 
  \key d\minor
  << 
    {
      \stemUp 
       a'16[ \startBarre "III" bes d' bes f'8] \stopBarre
    } 
    \\ 
    {
      \stemDown d2
    } 
  >>
}