Pagenumber in the side margin

You can put arbitrary code into the fancyhead definitions. Here is an example which puts page numbers approximately to the middle of outer margins (for the book document class).

documentclass[a5paper]{book}
usepackage{showframe}
usepackage{lipsum}
usepackage{fancyhdr}
fancyhead[RO]{%
  vbox to0pt{%
    vskip0.5headheight
    vskipheadsep
    vskip0.5textheight
    hbox totextwidth{%
      hskiptextwidth
      hskipmarginparsep
      thepage
      hss
    }%
  vss
  }%
}
fancyhead[LE]{%
  vbox to0pt{%
    vskip0.5headheight
    vskipheadsep
    vskip0.5textheight
    hbox totextwidth{%
      hss
      thepage
      hskiptextwidth
      hskipmarginparsep
    }%
  vss
  }%
}
fancyfoot{}
pagestyle{fancy}
begin{document}
  lipsum
end{document}