I know it's easy to come up with ideas but often much harder to implement them with the available technology, but one way this could be addressed is if the word "Banned!" appeared in red font for permanently banned users (and stay in black for others).
On the one hand it wouldn't do anything for the BU list, but people usually wonder about this kind of thing when running into the user's posts on a thread, not by checking the list. And if you're wondering about a user who's been MIA for a while, you can always use "Search" to find one of their posts. And then you'd see their status (banned or banned
Just a thought.