Maybe something like this?:
In your controller, when the user first makes the request, call the method that is going to execute the different operations, since these are blocking, wrap the DB calls in a given actor, then instead of calling a simple method to get the result, you should be able to ask the actor to do the operations, this ask
method (cf Akka doc) will return a future.
This future would then be sent to a given actor that is holding references to the different connections (meanwhile the controller returns a Result, Ok...), this actor should then listen for the state of the future, in case it's successful (onSuccess), push a message using the appropriate connection to the appropriate user. The out enumerator can be retrieved and saved when first creating the WS connection, a basic approach that comes to mind is that you can keep them in a map with the user id as key. You can then use these enumerators to push messages to the clients.