[Wim Lewis]

Just dashing off a quick reply here, but you should be able to do something along the lines of

tell application "OmniGraffle Professional 5"
  tell document "Some Document"
    repeat with ACanvas in every canvas
      repeat with AGraphic in every solid of ACanvas
        if (the text of AGraphic ≠ "") then
          set the font of the text of AGraphic to "Benton Sans"
        end if
      end repeat
    end repeat
  end tell
end tell

You could add other conditions to the "if" statement, if there are other graphics you don't want to modify.