Babybear
I think you raise the same somewhat circular debate.
You are required to have an instructor's sign off for any differences - note instructor not examiner - that much is certain.
Could you handle the controls to gain some tail wheel experience under the supervision of a PPL who is not formally instructing you. Who knows and maybe who cares. Is he instructing you or isnt he - what would his insurance company think if they knew. The fact is he is in command and is responsible if anything goes wrong and he most certainly cannot be paid. You would probably find if you were friends with such a PPL you would become very good at tail wheel flying so when you presented yourself to an instructor for an hour of difference training and a sign off you breeze through.