You could create a customer-specific group - it can be configured to match them via their domain.
You would make this a private group and have an associated category visible only to this group, which you would mark as ‘watching’ to get notifications.
Ah yes, thing is, I’d ideally like to keep it a bit secret from them
Creating a group wouldn’t be a problem, but I’d prefer it if I don’t have to force them to deviate from the normal behaviour and just participate in the forum (and are then amazingly well helped).
By listening to the post_created events you can check the user and create custom notifications in a plugin.
Then you can create a group which is only visible for “Group owners and admins”. And you can view all member activities in /groups/[the-company-group]/activity/posts.