The lateral head and the medial head of the triceps work in almost the exact same manner and extend the arm at the elbow. While changing the position of the upper arm or the forearm will result in the lateral head of the triceps being activated more or less in extension of the elbow, it is still simply extension of the elbow. And there is potential for elbow and shoulder injury if you are exceeding the limit of stability for these joints during weighted movement.
Isolated triceps extension added to the exercise program at the end of a resistance session can potentially enhance the development of this muscle. I would use equivalent biceps/flexor development that is reasonable to avoid strength imbalance between the two muscle groups.
And I would spend some time exploring the biomechanics of any exercise that I intended to have a client perform.