Once inside the jet stream altitude 7-15 km is it all that turbulent or is there a way to enter it smoothly for an ultralight plane or a powered glider? More info
Height is not the barrier. People have flown off Mount Everest - see http://abcnews.go.com/Travel/paragliding-everests-peak-kayaking-indian-ocean/story?id=21097292 as one example - though not when the jet stream is blowing over/around the mountain.
The problem is at the boundary layer between jet stream and none jet stream air when the aircraft only have air speeds of 20-50mph. The mixing of air, and the sudden hit of jet stream air speed will be well in excess of the gliders max air speed.
It has however been done in a hot air balloon. http://www.businessinsider.com/richard-branson-on-hot-air-ballooning-2014-10?IR=T