I think for very brief shots they could have gone the LIS route and stuck a model in front of a painting and managed to get away with it. As a child I always thought of Space 1999 and UFO as special effects leaders, but they came a full five years and more after TOS. Another point to ponder, Space 1999 sometimes tried to cheat. More than a few times they'd land an eagle model with the picture of another eagle angled or shrouded amongst rock or trees in the background. In a few instances they did a poor job, with an obvious picture propped on a launch pad. But if the shot is brief enough and done as well as possible, I'm sure they could have used similar effects strategies for TOS and pulled it off.