Thanks, Sam! Finally have some time to try to think about this. I think I understand what you’re saying. It does mean I proceed with my alternative plan:
Am I right in understanding that with your proposed improvement, messages sent to and from the site_contact_username would also include the site_contact_group_name? If so then this definitely solves the OP and I’m happy.
I will eagerly await the improvement, and in the meantime create a new helpdesk user and set that as the site_contact_username. I will keep an eye on that user’s messages and manually make sure they include the helpdesk_team group I will use as the site_contact_group_name when this is implemented.