how's the turtle? this thread is good reading.
I will say that i dont know much about turtles. I am a dentist by profession with a love for herps, insects, etc. So when i was looking at your turtles pics, the spines on the shell reminded me of teeth (i dont know why, but they did). Anyway, those of you who have ever had a toothache may remember that the dentist tapped on the tooth in question. It is not because he wanted to torture you, its bc your response to the tapping is diagnostic for us. In other words, pain is diagnostic. So what about the turtle?
Well, I would think if if the lesion is shell rot, the nerve endings will be dead (similar to a gangrene in humans). The turtle will have no response to a painful stimuli when the stimuli is applied to the lesion.
If it is bc the turtle is rubbing, then i am thinking the turtle will probably have inflammation due to trauma. This will cause a heightened response to a painful stimuli.
the rest of the healthy spines will act as a control.
so now you need to find a way to invoke a painful stimuli without causing lasting damage or infection. I am not saying stick a knife or a needle in your turtle. I am saying i dont know how to do it. but whatever it is, it needs to be small enough to only test the area you want. if you penetrate a lesion AND the healthy tissue, then the test is no good. and i know the turtle wont say "ouch" but you know your pets behavior best. you can probably tell if its in pain.
so i dunno, what do you guys think? i mean, its not conventional but maybe it will give you more clues.