The way I do it is to have (in
i-violin1.ly)
violinIFirstMov = {music}
violinIFirstMovBreaks = {s1*whatever \break s1*somethingElse \break}
Then in
violin1.ly
\score {
<< \violinIFirstMov \violinIFirstMovBreaks >>
}
<other movements>
In
score.ly, I just leave out all the xxxBreaks variables (I have never
had the need for explicit breaks in a score, but if you do, you can
always add another variable for that). Alternatively you could try
using \tag, but that still requires you to make the music messy with
manual breaking information. The manual counting of the spacer rests is
a bit tedious, but apart from that it works well. (If you use the new
page-turn-page-breaking algorithm in the parts, it cuts down hugely on
the number of manual breaks you need).