Trying to control chord symbols (shorter)

[rif]

I asked this a couple days ago but haven't received any answers, so I
thought I'd try to distill it down to a shorter question.

The two utterances here seem to produce the same set of pitches:

 \new Staff {
      \chordmode { c1:13 }
      \relative c' { <c e g bes d a'> }
    }

If I define an accompaniment track via 

  accompaniment = \chordmode { c1:13 c:13 }

the accompaniment symbol shows up at C^{9/add 13}.  What do I have to
put in chExceptionMusic to control this?  I've tried

   <c e g bes d a'>1-\markup { \super "13" }

as well as many other variants, but none of them see to work.  (I am
already successfully using chExceptionMusic to control some other
chords.)

Any help is appreciated.

Cheers,

rif