I am still wondering about how to upload legacy passwords to the device.

Initially I've considered using a bootloader, and that would also allow me to update the firmware, but this approach has its drawbacks: It requires installing libusb drivers and avrdude prior to the the first use.

Then I thought on an alternative solution: Use the LED states (lit/unlit) as clock and data lines, for instance:

NUM LOCK - DATA 0

CAPS LOCK - DATA 1

SCROLL LOCK - CLOCK

Then at every change on SCROLL LOCK I could sample 2 bits of data. Doing the math, for 16 passwords of 12 characters with a 6 bit wide character set I would need 16 * 12 * 6 = 1152 bits or 576 state changes. If I could do 20 changes per second (it's a guess) then it would take me half a minute to upload all the passwords.

Not bad!