The plant I would recommend is Comfrey, the Bocking 14 cultivar. You can buy root cuttings from the Garden Organic catalogue. The plants rapidly grow and can be divided and grown on to get many more plants. I wouldn't use them in your 10x10 area though as they would swamp everything else - they grow up to a metre tall. We have dotted plants around the fences, just on the other side, close to the fence. This way the sheep can graze the top growth as it peeps through the fence, but not destroy the roots.
Comfrey is an all round healthy plant, for people as well as sheep, and brings up minerals from deep down with its long roots. It really needs to be grazed directly from the plant rather than picked and fed, as it wilts rapidly and is then not palatable.
I agree about a willow hedge. When the plants are larger and need to be cut back, whole branches can be laid in the pastures and the sheep will strip the bark and any leaves, getting to wonderful minerals and vitamins under the bark. Willow is a wonderful winter tonic, and provides interest when feeding hay.