Again it turned out to be quite simple (although the underlying idea may be more complex);

The ServiceResource just forwards the call to the Service.

MyRequestService creates a MyJsonResultCompletableFuture (CompletableFuture implements CompletionStage) and delivers this to the Mutiny Uni with method completionStage(). Another possibility would be using;
Uni.subscribe().asCompletionStage()
The resulting Uni is returned to the ServiceResource.

Finally MyJsonResultCompletableFuture blocks the call from MyReactiveServiceResource / MyRequestService waiting for a completionStage. Method ready() accomplish this stage and returns the MyJsonResult to Mutiny (acting like some kind of callback).