
Haven't had a dog in that situation, but I do live with a perpetually bored/frustrated dobie/rotti X - she is my brothers, and is an incredibly energetic dog - but only gets 10 mins a day, if that, of walking, and is ignored most of the rest of the time. Drives me nuts.
To keep her occupied I give her a giant kong stuffed with a mix of crunchy peanut butter, mini kibble (the puppy version of her normal food), and a few snapped gravy bone treats mushed in there as well. It goes in the freezer last thing at night, and lasts a while the next day, keeps her busy. Treat balls are also a good idea, so that the dog has to work for its food - don't know how well that would work with the joint troubles though. An alternative could be a "molecuball", these don't roll around as freely as the regular treat balls so in theory your GSD could nudge it around whilst lying down.
Only other thing I can think of that *might* help is Nzymes - go to nzymes.com, they are a dietary supplement that have helped a lot of old, aching dogs as well as young ones and all sorts. first4pets.co.uk sell them in the UK. good luck!