High Availability and other replication technologies allow read-only replicas to exist independent of the primary database. For example, the Read Scale-Out provided by Azure SQL: https://docs.microsoft.com/en-us/azure/sql-database/sql-database-read-scale-out

This allows the main OLTP database to be unaffected by large OLAP-style queries.

The BC Service Tier would need to be able to be put into read-only mode, and issue the appropriate ApplicationIntent=ReadOnly command to the database. It could then provide services such as OData but using a read-only replica instead of the primary Read/Write database.

The only issue would be session tracking, but perhaps a read/write multitenant database would help here (or perhaps a second database link back to the original OLTP database).

Power BI could currently query the SQL tables, however this loses the benefit of user-security, flow fields and queries from within the BC system.