No, I never did find a solution. I dug around in the source, and found that the plugin itself computed sha256(private_key + content), not b64(sha256(…)), and stopped there. It seems like this plugin doesn’t quite work as described, but maybe others have had better luck.
To get arouund using this plugin, I my python script that prompts for a password for every post. If you’re interested, it’s here: https://github.com/benjaminaschultz/pypress
