Changing *displayed* barnumber

[Urs Liska]

Hi all,

I want to implement a function to display a modified barnumber in a box.
This is (can be) used in sketch edition to indicate that the barnumber
of the printed sketch doesn't match the one in the final composition.

This is what I came up with so far:

\version "2.19.37"

\layout {
  \context {
    \Score
    \override BarNumber.break-visibility = ##(#t #t #t)
  }
}

offsetBarnumber =
#(define-music-function (offset)(integer?)
   #{
     \override Score.BarNumber.stencil
        = #(make-stencil-boxer 0.1 0.25 ly:text-interface::print)
   #})

\relative g' {   
  \set Score.barNumberVisibility = #all-bar-numbers-visible
  \bar ""
  \repeat unfold 12 g4
  \offsetBarnumber 2
  \repeat unfold 12 g4
}
 
\offsetBarnumber will create the box.
But I don't have a clue how I can make the barnumber *print* a barnumber
that is offset by the argument. I do *not* want to change the
currentBarNumber as I would consider this semantically wrong.

Any hints or solutions appreciated

Thanks
Urs

PS: I don't seem to find how I can use the newer syntax for \override so
I don't have to use #{ #}
(override) is not defined so it must be some different approach