Atmel application note
"Migrating from the B to the New D DataFlash Family"
"The “D” family offers sector lockdown for secure code and data storage. The Lockdown mechanism
allows each individual sector to be permanently locked so that it becomes read only. Once
the sector is locked down, it can never be erased or programmed, and it can never be unlocked."
It uses a SPI interface very similar to the SPI interface of a SD/MMC card.
Alas, I think the details of its protocol are different enough that it can't be directly connected to a standard SD/MMC slot as if it were a standard SD/MMC card.
And so I don't think you can pop it into a standard off-the-shelf SD/MMC to USB adapter.
And so it looks like you'll need a custom-programmed microcontroller, as mjh2007 suggests. In which case, you might as well program that microcontroller to enforce "write-once" requirement, and use any random easily-available non-volatile memory.
Still, practically all software has bugs, and it might be nice to use this relatively easily-available Dataflash (Newark and Digikey have them in stock now) so you don't have to worry about some bug in the microcontroller accidentally overwriting your data.