I hadn't thought of the treadmill and stocks but it's a great idea. If his hands and head are secured then as you say no need for roped balls. 
It was a way of keeping him exercising and would also work if you had a rowing machine or cycle trainer - anything that he had to work to make it go rather than the machine making him work. The speed he makes the machine work would increase he electrical resistance therefore if he slackened off the full power would be released to his cock. 
Much more simple and easier to have his balls locked in the stocks together with his head and hands and just turn on the treadmill. Sorry, got carried away there :)