Page number offset and placement

[Hilary Snaden]

Is it possible to (1) allow the page-turn-breaker to behave normally 
but, if the first page number is 2, decrement all of the printed page 
numbers by 1, (2) place the odd page numbers in the top left corner of a 
page and the even numbers in the top right corner? This is for a case 
where a part will fit onto 4 pages, but only if the page layout is LR+LR 
rather than R+LR+L. (I seem to have a few of these.)

I've got this far:

oddHeaderMarkup = \markup {
  \fill-line {
    \line {
      \on-the-fly #print-page-number-check-first
      \fromproperty #'page:page-number-string
    }
    \center-column { \fromproperty #'header:instrument }
    \null
  }
}
evenHeaderMarkup = \markup {
  \fill-line {
    \null
    \center-column { \fromproperty #'header:instrument }
    \line {
      \on-the-fly #print-page-number-check-first
      \fromproperty #'page:page-number-string
    }
  }
}

But this adds the instrument to the header of the first page (which I 
don't want as it's already there) and I don't know how to decrement the 
page number before printing it.

--
Hilary Snaden