Maybe each province can be assigned a number of workers, and each governor order those workers.
The governors would also write a list of events where he/she would allow some or all of his workers to be sent off on a national project e.g. if a new resource is found.
so lets say I am assigned 5 workers. I would tell the DP to irrigate here, railroad there. Then I might say if:
A new resource is found
An existing resource is lost, and we need to connect another one
A rail or road has been bombed, and needs reconnecting
A railroad network in a nearby province is being built
A nearby province needs emergency fortresses
A city needs an urgent population boost (e.g. if it is under siege, and needs to reach pop 6)
etc.
I may give specific instructions for each event, e.g. for when a nearby rail network is being built, I may say - "send three workers, leaving two to work on the cities with the lowest food surplus. reassign me another 3 workers when finished"
If we lost our source of oil, and the nearest one is over some moutains, through the jungle, I may say - "Send all my workers to reconnect the oil. reassign me another 5 workers when the connection is complete"
we would decide how many workers a province gets by seeing how much work the province needs to reach full productivity.