Okay, it seems like groups has to be the way to go. FWIW, someone else has just had the same problem. I’ll look at editing the howto to say that the category option will really only work if the forum is closed to public registration/only users will be company staff.
With groups I still have two concerns.
One is if a user can’t access their account, but also has email notifications for PMs turned off. I assume they won’t receive an email reply to their email in? We’ll just have to be aware that it’s a possibility. And a likely one - most users won’t email unless they can’t access their account.
Second, is that I’d love it if messages were more prominent. Making it so that they’d appear in the latest/unread pages would be great. If it doesn’t already exist I might see about making a plugin/theme component to do that myself.