\version "2.16.1"

% log process music
logeng = #(lambda (context)
            `((process-music .
                ,(lambda (trans)
                   (ly:message "~A - ~A - ~A"
                     (ly:context-name context)
                     (ly:context-property context 'currentBarNumber)
                     (ly:context-property context 'measurePosition))
                   ))
              ))

% some notes
music = \relative {
  c4 e g a
}
postmusic = { \music c''1 }

\score {
  % Staff, CueVoice and Lyrics log every process-music call
  % It should be equivalent, if the notes past this cue-section 
  % are given within or after the SimultaneousMusic passage
  <<
    \new Staff \with { \consists \logeng } {
      <<
        {
          \voiceOne R1
        }
        \new CueVoice = "cue" \with {
          \consists \logeng
        } \relative {
          \voiceTwo \music
        }
        \new Lyrics \with {
          \consists \logeng
        } \lyricsto "cue" \lyricmode { la la la la la }
      >>
      % if the post notes are given after the Simul. passage, notes are squashed to a chord
      \postmusic
    }
    \new Staff \with { \consists \logeng } {
      <<
        {
          % here everything is fine
          \voiceOne R1 \postmusic
        }
        \new CueVoice = "cue" \with {
          \consists \logeng
        } \relative {
          \voiceTwo \music
        }
        \new Lyrics \with {
          \consists \logeng
        } \lyricsto "cue" \lyricmode { la la la la la }
      >>
    }
  >>
}

\score {
  <<
    \new Staff \with { \consists \logeng } {
      <<
        {
          \voiceOne R1
        }
        \new CueVoice = "cue" \with {
          \consists \logeng
        } \relative {
          \voiceTwo \music
        }
        \new Lyrics \with {
          \consists \logeng
        } \lyricsto "cue" \lyricmode { la la la la la }
      >>
      % if the post notes are given after the Simul. passage, notes are squashed to a chord
      \postmusic
    }
  >>
}