What about something like this? It's a simple Scheme engraver which replaces repeated chord-name text with a slash. (For the slash, I just removed the dots from the repeat markup which Robin posted in the other thread.) It's pretty rough, but it works with your last example. I don't have a more extensive example to try it out on to see where it might break down!
\version "2.15.24"
simile = \markup {
\rotate #90 \translate #'(0 . 2) \beam #2 #-1 #0.5
}
test =
#(lambda (context)
(let ((text #f))
`((acknowledgers
(chord-name-interface .
,(lambda (engraver grob source-engraver)
(if text
(if (equal? text (ly:grob-property grob 'text))
(set! (ly:grob-property grob 'text) simile)
(set! text (ly:grob-property grob 'text)))
(set! text (ly:grob-property grob 'text)))))))))
harmonies = \chordmode { c1 g e:m e:m e:m a:m bes c }
chs = \transpose c' c' {
<c e g bes des' >4 q q <c e g bes dis'>
<c e g bes dis'> \repeat unfold 3 { <c e g bes dis'> }
<c e g bes>1
}
<< \new ChordNames { \set chordChanges = ##f \harmonies } >>
<<
\new ChordNames { \chs }
\new Staff \transpose c c' { \chs }
>>
\layout {
\context {
\ChordNames
\consists #test
}
}