I haven't seen it done before, but why hasn't there been a client that encapsulates all features of LJ (including comments posting) there doesn't seem to be a "post/read-comments" defined in the protocol list (I've only browsed it very quickly).
Basically I was intending on programing a complete interface to LJ; ie remove the web-browser interface; just a simple client that keeps a backup of everything you post, everything your friends post, and comments written to/replied when needed. My thoughts were basically to decrease bandwidth usage on LJ's server side, which could in turn speed things up for everyone. It also helps a great deal if I can get raw data when writing the MIDP client, and could also provide for some nice stats on the user end of things :)
~shrugs~ just a thought though ^_^