Space based ADS-B tracking was already demonstrated in the fall of 2015 by a small Danish company on a very modest Cubesat called GOMX-3 launched from the International Space Station. It worked pretty darned well and returned good data until it reentered about a year later. Expect global ADS-B coverage to be a commodity in very short time, and costs to be very reasonable.
As a curve ball broadly inline with the same topic, folks are also experimenting with very small floating solar powered ADS-B receivers with similar goals.