LiveJournal's RPC backend does not allow usernames in postids to contain dashes instead of underscores, but does allow them when retrieving a list of posts via Blogger API (when the dashed username is used as blog key). This has to do with Interface/Blogger.pm having:
die "Invalid postid\n" unless $postid =~ /^(\w+):(\d+)$/;
The problem is you can use your dashed username to retrieve the posts (when setting up the blogging client it will be percieved as working by the user), but not to retrieve a concrete post or delete a post.
Either it should reject the login with dashes (implying that only an underscore might be used) or it should allow both for posting/deleting entries (i.e. when the journal name is used to retrieve entries). Or there should be a FAQ entry about it.