\version "2.13.40"

\relative c' {
  \override TextSpanner #'(bound-details left text) = "II"
  d4 fis\startTextSpan a b\stopTextSpan
}

#(define (format-textspan engraver event)
(let* ((context (ly:translator-context engraver))
       (moment (ly:context-current-moment context)))
  (display event)
  (display "\n")
;  (display (ly:event-property event 'bound-details))
;  (display "\n")
  (display
    (ly:context-property context 'TextSpanner )
  )
  (display "\n")
  (display "\n")
))

\layout {
  \context {
    \Staff
    \consists
    #(list
      (cons 'listeners
       (list
        (cons 'text-span-event format-textspan)
    )))
  }
}