I've also found the trapeze bar on a pilates thingummy to be very good - painful, but good. Not many gyms have pilates equipment, though. Anything that maintains that stretch, really.
I think that the issue with cording is that no-one really knows why it's happening (other than due to injury of the lymphatic system) and what it really is so it is convenient to ignore it's existence. Different reputable sites I've looked at quote the incidence as between 5%-85% of women suffering after axillary node clearance so it seems pretty clear that it's neither understood, nor well reported or researched. I have been told that it's a one-off thing that can be resolved and, that it's a forever thing that can only be managed. It seems that, while some have permanent relief, there's enough of us who inconveniently remain with permanent issues. There also appears to be two types of cording - the type that @zoffiel had originally that can be "popped" and the type I have (and possibly zoffiel has now) that is more of a web-like structure and will not pop. (Cording is actually called Axillary Web Syndrome.) Mine certainly involves the scar tissue from the mastectomy and things are tight under the skin around the scar. I have laser therapy and some sort of suction cup machine therapy to loosen the skin from the underlying scar tissue, as well as massage.