Subscription handling using own grpc service

With `proxy.grpc` we can already invoke existing grpc service without changing server code. However, it doesn't seem to be possible to use existing grpc service for Subscription. We still have to create another grpc service that inherits from `AppCallback.AppCallbackBase` and implements own serialization/deserialization in `OnTopicEvent`. Is it possible to use own grpc service as a Subscription handler as well?