balloon text applied to spanners?

[Jeffery Shivers]

Hi,

Is it possible to engrave balloon text to slurs, hairpins, etc? This
doesn't work:

\version "2.19.56"

music = {
  \new Voice \with { \consists "Balloon_engraver" } {
  \balloonGrobText #'NoteHead #'(-1 . -1) \markup { bla }
    c'4
  \balloonGrobText #'Slur #'(1 . -2) \markup { not engraved }
    d'2( c'4)
  }
}

\score {
  \music
}

Since footnotes can be applied to spanners, is there a small remedy /
alteration that can be made to the balloon text engraver to accomplish
it as well?

Best,
Jeffery

-- 

Jeffery Shivers
 jefferyshivers.com
 soundcloud.com/jefferyshivers