I've spoken to quite a few veteran pullers about this as my Lab Earl had a TPLO done on his knee 2 years ago and they all felt pulling was a fantastic idea.
The key is to start very very slow with the dog and do a lot of conditioning with drag weight to help them build up muscles in their rear first. Any sort of light weight resistence work is great for helping these guys keep their muscle tone in their injured legs.
I'm planning to order a harness for Earl in the next few months so I can begin working with him. I'm not sure if I will ever get him into competition pulling but I have seen huge benefits in conditioning and muscle tone with Kayden doing drag weight training a few times a week.
Look beyond what your own eyes see