Is it not the case that anyone at all can ask for the TCP, and anyone's bookings can be "linked" (such as it is), it's just that the second part of the process (the submission to Seating Control for the seat to be assigned) is either not done or actively ignored by Seating unless the pax is Emerald (or with infant it now seems, per your comment)? TCP in itself is nothing magical. The "magic" happens in the all-powerful Seating Control department.