Using terminal-notifier the script shows persistent notification center messages for all flagged OmniFocus tasks. Possible, in the notification center preferences one needs to configure the OmniFocus message properly.

gistfile1.applescript

tell application id "OFOC"
  tell default document
		set today to current date
		set lstTasks to flattened tasks where ((flagged is true and blocked is false) or (due date is not missing value and due date < today)) and completed is false
		repeat with oTask in lstTasks
			set theName to name of oTask
			set theId to id of oTask
			my notify(theName, "", "omnifocus:///task/" & theId, theId)
		end repeat
	end tell
end tell

on notify(theTitle, theDescription, theUrl, theId)
	do shell script "/usr/bin/terminal-notifier -sender com.omnigroup.OmniFocus -title \"" & theTitle & "\" " & "-message \"" & theDescription & "\" -group \"" & theId & "\" -open \"" & theUrl & "\""
end notify