tweaking properties after line break

[Simon Bailey]

hi,

i'm trying to shift RehearsalMarks which occur at line breaks further to the
left, so they are more aligned with the key signature rather than with the first
note of the music in that line.

i thought it would work like this, but it sets the extra-offset property for all
RehearsalMarks.

my lily file:

%###
#(define (mark-callback grob)
  (ly:grob-set-property! grob 'extra-offset '(-3 . 0)))
\version "2.7.17"
\score {
  \relative c'' {
    \override Score.RehearsalMark #'before-line-breaking = #mark-callback
    \repeat unfold 4 c2 \mark \default
    \break
    c2 c \mark \default 
    c2 c
  }
}
%###

is there a ly:<function> which is the equivalent of \once \override?

thanks,
simon.

p.s: i tried subscribing to this list earlier today, and have received no email
requesting confirmation yet (almost 3 hours later)? are all new subscriptions
first forwarded to the list admin for validation?