It's been a while since I've done this, but basically
1. Create a Ticket Tag called 'Seat' and set Sub Tags for all seat positions
2. Create a Rule 'Ticket Tag Selected' to run an action 'Update Program Settings' to save the Ticket Tag Value as 'Seat'
3. Setup a rule 'Line Added to Ticket' to run an action 'Update Ticket Item Tag' using 'Seat' Value from step 2.
If you need step by step, let me know, and when I have time I will write it.