Before I go and crank up the debug logging on my server to try and figure it out, I'm hoping someone (i.e., Daniel) can answer the following question:
How does MarsEdit post edited entries back to WordPress?
Specifically, what XML-RPC message is it using?
For the back-story as to why I'm asking, here's a blog post explaining what I'm trying to figure out: http://dossy.org/2011/08/cross-posting-from-wordpress-to-livejournal-woes/
In the meantime, I guess I'll start logging stuff to figure it out, but in case it's faster to ask ... well, I'm asking. ;-)