Rajendra Gupta has a detailed article on working with PolyBase in an Availability Group:
In this 28th article for SQL Server Always On Availability Group series, we explore the high-availability for the SQL Server PolyBase(SSB) external tables using AG groups.
There’s a lot of detail in the article and it’s worth reading in conjunction with Nathan Schoenack’s post. Someday I’ll get to the blog post on my backlog around PolyBase and AGs, especially with scale-out clusters. Someday.