Okay, I'm not trying to spam or anything but I happen to run into this video which clearly prove that you can make npc throw grenades even outside of missions. Watch closely at 1:42 and 1:53, you will notice the African-American Saint with a hoodie continuously throw grenades at his enemies, and it's outside of a mission, which mean it is possible to make npcs throw grenade without scripted events.
Now I know that this thread was created a long time ago but since I encountered this video I have to post this so everybody won't give up on making npc tossing explosives around the city.