Lemmings,
You are right, that selection required should trigger at the press of the settle button, not payment.
My suggestion would be to add a rule for 'Line Added to Ticket' with a condition 'Ticket Tag !? Driver' and display a message 'Select Driver'
This way, every time an item is selected and a Driver has not been selected, it will pop up a message. As soon as a Driver is selected, the message no longer pops up.
Not the solution you want, but staff will get annoyed after a while and start selecting the Driver first.
If your not using the Fast Food Department Type, set the Driver Ticket Tag as the POS Default Ticket Tag in the department settings. This will list the Drivers on the POS screen first.