If anyone is interested, I wrote a Greasemonkey script that removes Delicious and Twitter cross-posts from your LJ friends page. It does not completely remove them, but collapses them into a single placeholder line. You are then able to click through to the post and see the content. In that way, it’s like an enforced lj-cut tag. The script is probably not aware of every single kind of cross-posting app out there, but it is at least aware of the few that seem to dominate my friend list. Additionally, it should work for just about any style you could be using because it homes in on attributes of the post content rather than relying on style-specific formatting.
Now I can go back to re-friending people (or, at least, moving them from the posts-I-care-about-to-crap-posts-ratio-is-too-low group to a group I might actually look at.)