Hi David
an Atmel engineer wrote back in response to my question:
"Thank you for your request. We have not heard of any of this case before, but as long has you keep the part within its thermal limitations there should not be possible to erase the flash. If you want us to investigate further, please provide more information about how this happens, when it happens and what kind of code and hardware setup there is."
There you go... PM me if you want his address.
D