Hello. Recently I tried to pass values from order on Shipment screen to Invoice and it was quite difficult.

The main problem that I faced that this value always overrides to empty string if you have more than 1 order on Shipment screen

Even if you try to override the Prepare Invoice method you can not easily set value to customer order number because some base methods that calls inside Prepare Invoice steel override this value. I made it in next way
 

I took the all body method from Prepare Invoice and before redirection to Invoice screen I set value from order customer order number if they all the same. And now it works fine