how to override dot-placement-list (fret diagrams)

[pls]

Hey all,

I would like to override some fingering information of predefined fret
diagrams.  The relevant information should be in the
/dot-placement-list/ of the fret-diagram-interface (see
http://lilypond.org/doc/v2.19/Documentation/internals-big-page#fret_002ddiagram_002dinterface).

But I couldn't find any example of an override of this information
neither in the Notation Reference nor in the Snippet Repository.

As a first step I tried to override the fingering of the fret diagram
for /c/ (predefined-guitar-fretboards.ly).  My goal was to change the
finger on the third fret of the fifth string from 3 to 4.  The following
override does not work.  Am I missing something?

\version "2.19.31"
\include "predefined-guitar-fretboards.ly"
<<
  \new ChordNames \chordmode { c1 | } %1
  \new FretBoards \chordmode {
    % overrides
    %\override FretBoard.size = #'2.2
    %\override FretBoard.fret-diagram-details.finger-code = #'in-dot
    \once \override FretBoards.FretBoard.dot-placement-list = #'(
      (place-fret 5 3 4)
    )

    % fretboards
    c1 |%1
  }
  \new Voice {
    c'1 |%1
  }
>>

Thanks for any hint!
Patrick