I vaguely remember that someone figured out how to write the signature bytes.
I have a batch of atmel chips where atmel (Can't get used to saying microchip) forgot to do so (correctly) in the factory. They are replacing the chips, so the bad chips are now available for "experiments" :-)
I have searched the forum, and the proof that this is possible is evident on the forum beginning in about 2005. People are reporting accidentally wiping their signature bytes.
The hardware DFU bootloader is also missing. Does anybody have a binary for that?