I'm making this add-on at the moment. It is pretty much completed.
That add-on used to cost $15... last time I checked.
Also, that add-on only checks if the username is a paid minecraft account. The title of this request may be a little mis-leading... and I won't be selling it under that name. If you read the description the op's given, it might make a little more sense.
The add-on I've made allows users to register on a server (you can define multiple servers), once registered it will send commands (speciifed in the AdminCP) to the server via Rcon.
Some screenshots may explain it better.