I can see that there's a good opportunity for fraud with someone having an Oyster travelcard for less zones, getting on there and just chancing it the majority of the time. If you're on an all stations train (as I expect most are?) then as long as you're checked once you've entered your covered zones, you'll go undetected as they'll just scan your Oyster and get the green light. They won't ask everyone where they got on, or have any way to know.
If you are stopped sooner than that, you can just say you couldn't buy your extension and pay there and then (rather than being given a PF which they can't). Or they have to investigate you over a period of time and prove that day in, day out, you do this - and when not checked, you never pay for your extension at the destination. How likely are they to do this? Possible more than most other TOCs, but probably still slim.
A permit machine wouldn't therefore help much, as you'd still keep quiet but know you have a 'get of jail card' to simply prove what you're claiming anyway. Only if you were found to have put in just 5p when you had money to pay the full fare (or as much as possible) could they perhaps do something, and even then I am not sure about the legals on someone just paying 5p when they had more money.
A permit with nothing to pay is pointless, as nobody COULD pay the right fare even if they wanted to. So you might as well not bother with one, unless the permit is simply to cover people who might get accused of joining the train elsewhere. I am sure that happens.
You know the real solution is to have the station staffed and the money emptied regularly to make it pointless to break into them, but that isn't going to be economically viable (but, hey, if London Overground ever did take over - chances are it would be).
Or, perhaps do a deal with a nearby garden centre to offer ticketing facilities (in the form of a TVM) which would at least mean there's a way to buy tickets during normal shop opening hours. It's been a while since I lived near there, so I can't remember how close the nearest shop is.