How to override VoltaBracket once?

[TaoCG]

Hi,

what I am basically trying to do is to override a property of VoltaBracket
once but I can't figure out where to place the command in the code. When I
remove \once it works fine. I know I can just revert the property but I am
still curious why it won't work.

Actually I have a coda sign in the alternative-part and I set its
outside-staff-priority to 0 to have it below the volta bracket but now the
volta bracket seems to be to far away.
The following snippet produces what I want but if I try to override with
\once nothing happens.

\version "2.13.52"


music =
{
        \repeat unfold 6 { c'1 }
}

frame =
{
        \override Score.VoltaBracket #'extra-offset = #'(0 . -1.5)      
        \repeat volta 2
        {               
                s1*2
        }
        \alternative
        {
                { s1*2 }
                {
                        s1
                        \once \override Score.RehearsalMark 
#'outside-staff-priority = #0
                        \mark \markup \musicglyph #"scripts.coda"
                        s1
                }
        }
        \bar "||"
}

\score
{
        <<
                \new Staff = "s" \new Voice = "v" << \frame \music >>
        >>
}
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