Mike Kelly submitted two patents in 1997 and 2000 detailing a tow to launch architecture. Dryden teamed up with KST in the late 1990s for Eclipse, which demonstrated the concept's fundamental soundness for existing lifting bodies in the subsonic regime. KST (or a descendent team after it went belly up) pursued the X-Prize, lost, and apparently reconstituted under new management. Not sure what happened to Mike Kelly, or who owns the patents.
In any case, is there any particular reason why tow-to-launch hasn't been explored more vigorously? Not sure what the cost of the Eclipse test was, but the follow on contract went for $1.2 million at the time. Isn't this the sort of potentially low-cost, high trial rate approach to a problem that produces breakthroughs?