\version "2.19.27"

verticalText #(define-event-function (text) (markup?)
    #{
      -\tweak TextScript.self-alignment-X #LEFT
      -\markup \rotate #90 #text
    #})

{
   c'2^\verticalText \markup \italic "This is a long markup" c
   c'2\verticalText "This is a long text" c
}

%%% modified from define-markup-commands.scm, line 701ff.
#(define-markup-command (rotate-by-anchor layout props angle anchor arg)
  (number? pair? markup?)
  #:category align
  "
@cindex rotating text

Rotate object with @var{angle} degrees around @var{anchor}.

@lilypond[verbatim,quote]
\\markup {
  default
  \\hspace #2
  \\rotate #45 #'(-1 . 0)
  \\line {
    rotated 45°
  }
}
@end lilypond"
  (let* ((stil (interpret-markup layout props arg)))
    (ly:stencil-rotate stil angle (car anchor) (cdr anchor))))

{
  c'2 ^\markup \rotate-by-anchor #90 #'(-1 . 0) "This is a long markup" c
}

#(define-markup-command (my-rotate layout props ang arg)
  (number? markup?)
  (let* ((stil (interpret-markup layout props arg))
         (stil-x-length (interval-length (ly:stencil-extent stil X)))
         (stil-y-length (interval-length (ly:stencil-extent stil Y))))
    (ly:stencil-rotate stil ang (/ stil-y-length -2) (/ stil-x-length -2))))

\markup {
    "ab"
    \my-rotate #90 "------|------"
    "xy"
}