The issue here turned out to be that I was not scrubbing the write-protect screw hole enough. I had to take the board out and scrub the screw hole with rubbing alcohol in order to get enough traction to clean the gunk I wasn't seeing. I had been scrubbing the board while leaving it in the case; apparently that wasn't enough. I finally got a success on flashrom --wp-disable after the fifth or sixth time cleaning. Thanks for the help!