The JPA Basic Model supported a read-only mode by setting the "supportsUpdates" property to false on the JpaSource. This feature was unintentionally omitted from the Simple Model.
This could be added to the MapRequestProcessor and MapRepository classes pretty easily (which would enable it for the Simple Model, the JBoss Cache repository, Infinispan repository, and the InMemoryRepository (where it would be worse than useless). However, I'm struggling to see where this would be used, especially since the database for the JPA connector is not intended to be shared.
If anyone needs to set up a master-clone configuration for DNA where the master DNA server has read-write access to the database and the clone has read-only access, we could add this pretty quickly.