thats a good question... My person opinion is, if its in question dont try it... Im not gonna risk losing my certificate and career as a commercial pilot just to give a student some good practical training... Now thats not to say that I dont think its good training..

Think about this though (not to sound paranoid), what if the owner of that property was out in his field when you were doing this approach and got your tail number and reported you to the FAA/FSDO... now not only are you facing busting the 500ft rule, but you could get a violation for reckless/dangerous operation....

but as instructors we both know that real world scenerio training can be very valuable if needed in real life.... Big dilemma....