How does Project get the final Selling Price from OM to interface to AR

Summary:

We are implementing project driven supply chain for our ship building client.
For the ETO scenario, they plan to create a sales order with ETO item with zero price (since they dont know what would go into creating the item and the price related to the same) and tag the project and task details in the sales order.

They want to do milestone billing. Since ship is a big vessel, they charge the customers as a part of ship building is completed. Could you please guide on how can we achieve this ?

How project will interact with OM and milestone billing will be achieved . There will be multiple billing milestones before the final finished good is shipped.