I discovered the reason for my failure to cross-post things successfully, by the way: I didn't realise (a) that DW also respected the default privacy settings of the LJ account as well as the privacy settings of the DW post itself, and that (b) I'd set my LJ account to post things as "private" by default (I remember doing this, now, because I had a habit of making stupid drunken posts at 2am that made me look like an idiot). So the function was behaving as it should, I just didn't quite understand what it was doing. Seems fine now.
Is it just me or does cross-posting from DW to LJ not work properly if you select "minimum security level: friends"? It seems to set all my cross-posts to "private", which is not what I want at all. I don't want to reveal my old LJ ID to any sod who happens to be reading, but I'm quite happy with my friends still being able to read my entries and also know this username.