About this proxy, you could use a custom action which would simply do what you said. Put this custom action just before the form action in your stories, something like this:
## ask for flight info
* request_flight_info
- action_fill_extracted_slots
- flight_info_form
- form{"name": "flight_info_form"}
- form{"name": null}
Now in the custom action, extract the values of the entity GPE (look into get_latest_entity_values) and simply return slot set events to set the slot origin and destination with extracted values. This way when the form will activate, it’ll get those slots as filled and will not ask for it, otherwise it’ll ask.