Hello! I recently added a blog post on how to replicate Master-Detail / Lookup relationship to the Opportunity Product object with Process Builder in conjunction with other components. Hope this helps with those who have come up with this request before as well, but had to find a workaround due to Opportunity Products not supporting custom child objects. Any feedback is much appreciated!

@Salesforce Flow Automation  @DEV450: Programmatic Development @Salesforce Certified Professionals 

wp.me