All are in the LJ.XMLRPC namespace of course:
These were gleaned from: code.livejournal.org/trac/livejournal/br
It seems only paid users can call addcomments.
This codebase is over a year old, it looks like they aren't using that repository. I believe there may be other methods. The iPhone app developercosysoftware_en some upcoming functionalities in his app that don't seem available from these methods. He claims to have gotten LJ to extend the XML-RPC interface, but won't disclose what these new functions might be.
The LJ support is useless. Is there another way to contact LJ devs? Anyone with XML-RPC client should implore LJ to update their documentation.