Hi!
According to the same spec sheet you referenced earlier it states " This pin maybe kept in this range at times other than programming. But it is not recommended." Note 2 on page 254. You are talking about being able to supply a voltage at 10 ma over a wide voltage range so again I would be back to suggesting an optical isolator or 7812 unless you know the zener you plan to use will tolerate more than 10 ma. Best, Ed!