I have to admit that I am not well versed on Comfrey. What I do know now is what I have read since I read your original post.
From what I can gather, Comfrey would not make a good animal feed because of the chemical composition. If it isn't good for Human Livers it wouldn't be good for animal systems either, and I think Deer would avoid it. But that would depend on how it tastes I suppose.
Also, I suppose it would be a fast growing plant after the first year, so I doubt deer would be able to keep up with growth. Unless you have tons of deer around.
Wikipedia has many uses for Comfrey in its website, and the directions on how to use it, including mulch. I suggest you do alot more reading on the internet regarding this plant before you establish it on your property. The FDA has banned this plant for human internal use, and probably with good reason.
There are other ways to take care of sandy soil. I have the opposite problem. I have clay soil. And Mulching, Sand or Perlite, Lime and Fertilizer is the constant answer here.