I am thinking A CG tow hook for the towplane, likr RC models do.
This way Glider can stall the tow plane while kiting, but not point its nose down.
I envision a tunnel for the tow rope along the fuselage top so it doesn't jam the rudder/elevator. The front end of tunnel/rope is connected to the fuselage at the CG.
Once the glider starts rising above the Towplane, the tunnel hinges upwards while the rope is still connected to the towplane's CG.
No moment is generated to raise the towplanes tail.

Dan