I played about with the LiveJournal cross-poster last night, but it got late, and I’ve only just tested it out properly. To my surprise it all seems to work.
The other reason was the use of lj-cut. The cross-poster automatically translates the WordPress “More” tags into “lj-cut”, which is all well and good; but often, where I feel it appropriate to use lj-cut (for those of you LJers reading this on your friends’ page), I still wish the entire post to appear in WordPress. I have now added “lj-cut” as a tag in my WordPress editor, so using that, it gets cut in LJ, but ignored in WordPress; while using the “More” tag will cause the post to be cut in both.
This is now looking quite good, so I have switched back to posting to LJ in public, and things should all start working again.