Normally when a non-persistable entity is not referenced anymore by anything that is reachable in either the server (including being explicitly added to the session by java actions) or the client, it would just get garbage-collected. Since you're saying you don't even have associations, I'm having a hard time painting the picture of how your application is looking like and what you want to achieve or if it is the right way to approach things. Maybe you could elaborate?
I managed to get a solution. Since webserviceAPI domain models are not accessible to create a relationship with persistent entity A, I created another non-persitent entity (D) in A's domain model and associated it to the persistent entity A. I copied relevant content from entity B into D via microflow.
Now the datagrid of entity B has entity D as the datasource via microflow.
This solved my problem. Thanks @Broek for the feedback about garbage collection.