Thanks for the response! I found a plugin to require approval for new posts, but here’s the catch; the site owners would like to approve not only new posts but also any subsequent edits.
I may be wrong, but I figure the best way to achieve this is to save each post submission into a clone of the posts table. After that, an administrator can approve the post, in which case it’s copied to the real posts table. So basically, any changes to posts or pages would need to be redirected to this “holding area”.
This could be done by digging in to the code and changing some stuff around, but then I’d lose the ability to upgrade WP (not to mention that such a hack wouldn’t be of much use to anyone but me). I’d like to make it into a plugin, but I’m not sure that abducting post submissions is possible. (Please correct me if I’m wrong!)