A missile only needs to get close enough for its sensors to take over for the final approach right? Transponder data should be quite enough for that, especially for a kc-46

Any of the methods i mentioned is enough to get missile close, except may be microphones as limited speed of sound means that the plane would have moved significantly from the observed position, though again even that would have allowed to put missile into the vicinity and in general direction.

Watching Ukraine videos there is new game in town though - relatively cheap IR cameras. Using IR, day or night, you can detect a jet plane from very large distances and just guide missile to the plane computer-game-joystick style.