Erm, actually, I wasn't assuming anything about user behavior. I was completely forgetting that we even had users, and just thinking about the technical aspects. :)

No, really, the whole client support thing wasn't taken into account in the crossposter design. It will almost certainly be updated at some point, assuming that someone has the time to code it. There will be certain features that would be nice to have that won't get put in for technical reasons. (For instance, taking your example, it would be nice if you could actually edit your crossposts at LJ and have the changes propagate back to the post on DW, but unless LJ wants to update their code, it's not going to happen.) But hopefully it will be something that works for most cases.