I got bored so I made this. It allows you to add users to the server whitelist file. Note that if you use this while the server is running, the file will be overwritten when the server is stopped. It does need some nicer commands/documentation, but whatever.
Oh it also caches the requests made to mojang's api, and I dont expect this to work on windows because of my hard coding of the caching file (/tmp/minecraft_whitelist_edit).