You’re correct that workflows and Data Quality Automation don’t currently support updating individual line item properties directly. That’s a limitation for many users trying to calculate things like Unit Discount dynamically.
To achieve your goal, you’ll likely need to go the custom quote template route. HubSpot allows you to create and modify quote templates using the Design Manager. You can clone an existing quote template and then edit the line_items_table module to add a new column for your custom “Unit Discount” property. This way, you can display the Unit Discount for each product line.
Now, as for the calculation (unit price × discount %), HubSpot won’t automatically compute that for you unless you’re using an external script to populate that field before generating the quote. So you’d need to calculate and populate that custom field—maybe through a custom integration or a third-party tool—so that when the quote pulls in the line items, it already has the Unit Discount values ready to display.
Long story short: yes, you should create a custom quote template, and you’ll need to manually or externally populate that custom line item property for now. Hopefully, HubSpot will expand line-item-level automation in the future!