how about this, as an alternative to going down the route of secure loaders and such. The firmware that's written to the chip is padded out to (sizeof(flash)-X

how about this, as an alternative to going down the route of secure loaders and such. The firmware that's written to the chip is padded out to (sizeof(flash)-X) with cryptographicly secure random bytes, where X is the size of a crypto signature block, which you use to sign the firmware. When you plug the thing into a computer without the sd card inserted(I plan on storing the password vault on an sd card, correctly encrypted), it dumps all of the flash via keyboard. Pipe that into a program that verifies the signature, via whatever means, and you can ensure the firmware has not been tampered short of hardware modification in the form of adding more flash memory/eeprom, as to do so would require compressing the existing code/cryptgraphically secure padding, as you would need to dump the original out to pass the verification. If you want to be even more secure, you can even replace the existing signature with your own, therefore ensuring that people can't change the version without access to your private key.