It might be nice to add an option to Face-to-face notifications where you can choose to send a message to users who have signed up for a session, but have not yet received manager approval. Included should be the option to send the message to the manager.
Currently, you have these options for recipients (screenshot):
An option for "unapproved" bookings might be nice in this list of Recipients options. Or is there another way to achieve this easily?