Ok, usei um Applescript para fazer isso depois de passar por esses sites:
tell application "OmniGraffle Professional 5"
activate
set theDocument to front document
set theCanvases to every canvas of theDocument
set canvasCount to count of canvases of theDocument
repeat with canvasNumber from 1 to canvasCount
set canvasNumber to canvasNumber as number
if canvasNumber < 10 then
set dd to "0" & canvasNumber
else
set dd to canvasNumber
end if
set name of canvas canvasNumber of theDocument to "Mockup " & dd
set canvas of front window to canvas canvasNumber of theDocument
end repeat
end tell