Is there a nodecomment equivalent/replacement for D7?
@Chris You started me down the right path, thanks so much! I've tested and can implement it this way without writing any custom code:  


Create 3 content types: Helpdesk Ticket, Ticket Internal Note, Ticket External Note.
Create a role called Helpdesk Volunteer.
Put a node reference field on Ticket Internal Note and Ticket External Note using the node reference URL widget.
Use the Content Access module (currently still in dev) to restrict view/edit/delete of the Ticket Internal Note content type to users in the Helpdesk Volunteer role.
Create a view with EVA that attaches to the Helpdesk Ticket content type, set to display both internal and external notes.  


Afterwards, it's just a touch of prettification and theming as needed. Add a file attachment upload field and a ticket category. Maybe implement messaging/notifications to have email notifications and reply-by-email. Fully functional helpdesk on D7!