In the end I used FME to push data to the Assignments feature service on AGOL.
No code was used.
Craig did mention above that you can use ArcGIS Pro to push to the feature service.
Another code-free workflow for batch creating assignments based on an existing feature service, would build on the "Identify features" tool (screencap below). However, since right now this feature supports only rectangular selection, the workaround would be to first filter the dispatcher's map (say, to show all assets in a specific zone), and then select them all and batch create the assignments.