At (say) 50mph the glider is covering 4400 feet per minute. At 50:1 it is losing 4400/50 fpm, ie 88fpm. Thus rising air at only 88fpm will enable it to maintain altitude, any more than that and it will be able to climb (or stay level and go faster at a less efficient l/d ratio).
That bit sounds perfectly feasible in theorey. The problems would be simply finding the jetstream, let alone finding it's edge, and getting the glider there in the first place. Those, I suspect, are pretty much insurmountable.
Surely of far more importance would be a way for airliners to find this rising air and stay in it enabling a considerable reduction in power to maintain speed. I have flown a Jetranger several times in strong ridge lift (canyon edge) for many miles at ludicrously low power settings so the theorey appears sound.
Have I got that right?