How to use line-break-penalty?

[Mark Knoop]

I'm trying to use the line-break-penalty property of the
paper-column-interface to set some preferred line-breaking positions
(e.g. to prefer a line break at the start of a variation in a set of
variations, or at a repeat sign). 

However I'd prefer not to _force_ a line-break with \break as I want the
source to be usable for different paper and staff sizes. (LaTeX does
this well with \linebreak[2], for example.)

I'm using:

\once \override Score.PaperColumn #'line-break-penalty = #-20

but it doesn't seem to be having any effect. Any ideas?

-- 
Mark Knoop