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