How is your hen doing now? It sounds like she is feeling a lot better!
I have a 4 month old pullet who just suffered a similar attack today. She apparently found a hole in the fence and ended up in the neighbor's yard, with the neighbor's lab mix. We didn't know until we heard squawking and then saw the dog with our chicken in her mouth. The pullet was quiet and very still, so we thought she was dead--the dog was actually digging a hole to bury her--when I saw Delphine's leg move. I yelled at the dog and she dropped the Delphine, who went running into the bushes.
Now she has wounds on her back very much like your hen's--a lot of feather loss on her back and tail and a bloody area on her back, but the wounds didn't look deep at all. She can walk, but her gait looks a little odd--kind of drunk/wobbly--and she was trembling for awhile. After reading here, I flushed the wound with saline, cleaned it with a foaming betadine type spray (Bac-D?), and put some bacitracin on it. Delphine is isolated in the coop with a heat lamp. At first she was cowering in a corner, but as it got towards bedtime she went up onto the high perch, which requires either a big jump or a little flying, so I took that to be a good sign. I don't think she's been eating or drinking, though. I put a bunch of dried mealworms right by her, and it doesn't look like she touched them (that's usually their favorite treat). I'm worried about internal injuries. I made her a vet appointment for the morning, mostly to get antibiotics (the vet is much closer than any feed store around here), but I don't have much faith that they really know what to do for a chicken.
I'd love to hear how your girl is doing. Are you going to keep her isolated until her back heals fully, or at least scabs?