Break within measure in polyphony

[Nick Payne]

I'm attempting to reproduce the engraving of a baroque prelude which is without barlines (although it appears to be in 12/8, as the bass voice has some dotted whole notes), and at one point the original engraving has put a line break two thirds of the way through a dotted half note in the bass voice. The solution given in the NR for forcing a line break in the middle of a measure (http://www.lilypond.org/doc/v2.17/Documentation/notation/line-breaking) doesn't work here - Lilypond puts a warning in the log "forced break was overridden by some other event, should you be using bar checks?", and ignores the forced break. Is there any way to force a break at this point? This example demonstrates what happens (I'm using 2.17.26):

{
  << {
  \repeat unfold 10 { c''4 }
  \bar "" \break
  \repeat unfold 10 { c''4 }
  }
  \\
  { \repeat unfold 5 { c1 }
  } >>
}