Exciting Stuff.
I love the idea of flying turbines; getting out of the slow air in the boundary layer close to Earth has the potential to harness so, so much more power. Taking this as a general aim rather than looking specifically at the jet stream, you wouldn't need such long tethers as those which worry theotherwill. However, there must be big issues in overcoming ensuring they stay airborne no matter what, plus the repercussions that long, strong, invisible strands stretching a thousand feet into the air would have for low-flying aircraft.