So I think I’ve finally found need to actually write my own plugin. Basically it is a “Moderate” button that will appear to certain WP users next to each comment. When clicked it will place the comment into a special moderated queue which will be visible to moderators, but not to the general public.
I’ll probably use Role Manger to create a moderator user class and assign them the capabilities of Subscriber, and an additional capability for moderation.
I think setting the background of a moderated comment to a light grey will be an easy way to distinguish between a moderated comment and a publicly visible one.
Now the only trick is to find the time to do it.