Ah I think I understand now.
So maybe the following feature:
Within an incoming you can select a ‘Custom object’ subscriber.
This presents you with one field, like so:
Where you can write your data with environment variables to start the route with.
Would that feature be the solution?
Workaround
A workaround would be using the storage subscriber or using a http subscriber to a fake storage or endpoint.
Then overwriting all data in the first Entity transformer.
Have a nice holiday.