The only time i ever really feel the lateral head get pumped is when i do standing tricep reverse pushdowns. I dont know if that is the real name, but essentially, you get the bar at the pully, as if you were gonna do a push down, except, you grab the bar with your palms facing upward and then pull the bar down through the extension.
My lateral head gets pumped from this, but the movement to me feels really akward and I think it hurts my elbow. I only do it occationally, but other wise, my lateral head never gets to pumped.
-Fatty