Line-breaking with non-aligned barlines - again!

[David Sumbler]

To recap:

   3 days ago I wrote (with this subject line) that I have a passage to
   set where one instrument (harpsichord) plays 8 bars in 4/4 time,
   consisting mainly of semiquavers (sixteenths).  It has to be played
   in strict time at its own tempo (4 = 108), which is quicker than the
   rest of the orchestra is playing.

   Although the harpsichord part must appear in the score, it is
   important that it is clear that it doesn't "fit" with the other
   instruments.  It starts at a common barline, but thereafter it does
   its own thing, so the positioning of the notes in the score is
   merely a suggestion of what is going on: in performance it won't
   necessarily fit precisely as it is shown in the score, but it is not
   expected to.

   By using scaled durations etc. I can get roughly the effect I want. 
   But is it spoiled by one thing: there is a 4/4 time signature in all
   instruments (including the harpsichord) at the beginning of the
   passage, but there are also a couple of further time changes in the
   orchestra only.  It is easy enough to prevent these from appearing
   in the harpsichord part; but because Lilypond synchronises
   everything vertically (just as one wants it to 99.99% of the time)
   it ruins the effect.  This is because there is a large gap between
   successive notes in the harpsichord to allow for the time signature
   displayed in all the other staves.

I have not had any suggestions as to how I might get Lilypond to set
the harpsichord part without regard to the spacing on the other lines;
this strongly suggests that, as I suspected, it can't be done! 
(Although if anyone knows otherwise, I'd be glad to hear about it.)

Meanwhile I have been trying another approach, which is to set the
harpsichord part on its own, with line breaks at appropriate points
(within bars) to suit the bar-end breaks in the orchestra score.  I
then save each line of this as a separate eps. file.  In the main score
I have a dummy stave above the string parts, and I attach the relevant
file to a dummy note using

  \once \override TextScript.extra-offset = #'(-10 . -6)
  a'1*2
  ^\markup \epsfile #X #120 "No16Harpsichord1.eps"

The actual values required are found by trial and error.

This promises to work, except for one problem.  For some reason that I
can't figure out, the spacing in the orchestra lines gets affected.  As
I increase the size of the image, the orchestra score either extends
into the right-hand page margin or expands horizontally to force one or
two bars on to the next page.  There doesn't seem to be any direct
relationship between where the right-hand edge of the image occurs and
the point in the music of the main score that lies immediately below
it.

I have tried using \textLengthOff, but this does not help.

Can somebody tell me what might be going on here, and perhaps how to
get around the problem?

David