What boggles my mind about positioning flights is that it's often the smaller airlines that are the most prolific.
For example this week alone Cityjet have had two empty positioning flights out of Dublin ( one to Edinburgh and one to Cardiff ) to be in the right place to operate scheduled flights. That's at least 20,000EUR gone, flying empty seats around.
I understand that small fleets mean less flexibility but you'd think the smaller airlines would have adopted the OP's idea by now to try and claw some revenue back.
Though they probably find some way to blame positioning flights on their customers anyway...