I recently just happened to buy one of these TDS820 for some testing. I did a backup of the firmware and nvram (v2.03e) and unlocked the FFT (option 2F). I attach here if anyone needs in the future.
Unlock option 2F (FFT):
unlock nvram switch on side first, send commands over gpib, then lock, reboot, done
PASSWORD PITBULL
WORDCONSTANT:ATPUT 327689,1
commands I used to get the firmware/nvram:
./tektool -r NVRAM_all.bin -b 0x04000000 -l 0x100000
./tektool -r firmware.bin -b 0x01000000 -l 0x400000