I've just completed a tutorial explaining how to implement support for a specific protocol inside SIP Communicator.
For the purpose I had recently implemented in SIP Communicator support for a dummy protocol called Gibberish. Basically all that Gibberish does is let users add all the contacts they want to their contact list and then echoes messages and typing notifications back to them.
It is now actually possible to create your own Gibberish account in SIP Communicator (any user name and password would do) and use it to test its features without even needing a network connection.
The "protocol" has the advantage of being extremely simple and I've therefore used it as an example in the tutorial. Apart from that you (sc developers) could also use it in order to test your services. Yana for example is already using it for testing new features in the UI tests and she seems to find it useful.
So anyways, if any of you guys are currently working on an implementation of a protocol for SIP Communicator, you'll probably want to have a look at the tutorial.
I'd be glad to have any feedback so in case you should have any comments or suggestions, don't hesitate to send them over.