Cannot write lock byte on Arduino Pro Micro
Meanwhile, I managed to solve this problem.
The reason for the verification error is that the two most significant bits of the Lock Byte are read-only on my variant of the Arduino Pro Micro. Which means that, if you (try to) write a value of 0x00 = 0b0000 0000, it will result in a value of 0b1100 0000 = 0xC0 on the hardware.
The solution, then, was to simply ignore those two bits.

&gt; avrdude -p m32u4 -P /dev/ttyUSB0 -c stk500v2 -b 115200 -U lock:w:0xCF:m