Have you got any experience or tips for walking a dog from a manual or power assisted wheelchair?

Get yourself a side-release buckle strap and a Flexilead leash. Loop the belt through the flexilead handle and then secure the belt around your waist. Connect your dog’s collar or harness to the lead. Assuming your dog is trained to heel, you’ll have no problem keeping the flexilead loose (not locked to a fixed length) as you push your chair. If you need to control the dog, you can simply wrap your hand around the lead.