All that about the 'revised' route being illegal because it hadn't been approved is nonsense. It is not setting up a new scheduled service to anywhere new, just planning a one-off unscheduled stop en-route. If such a situation were not allowed, how would bizjets and GA aircraft ever be allowed to operate? You file a flight plan before a flight, it gets approved by ATC, therefore the flight is approved (slight oversimplification I know).
Actually it isn't nonsense. Once you leave the comfort of the EU you have to get approvals for each flight. Some countries say that filing the flight plan constitutes getting approval, but many (including Turkey) don't. You must get your overflight/landing permit first and place the number in the remarks section of the flight plan. It is for a specific destination - some countries even for a specific routing. Getting this approval can take anywhere from a few minutes to weeks (India, anyone?).
I have often used the procedure of taking off on my original flight plan and then once safely back in civilisation "discovered" that I needed to divert.
