This can be a nice idea for some tech tree. Actually there is one which has a unit named Trebuchet, but I'm not sure it does what you have in mind.
The Megapack (default techtree in MegaGlest) will most likely not see any larger modifications such as this - this is a design decision because it is considered because any such changes can effect in loss of faction balance. However, you can, without much effort, create a copy of Megapack and add an existing Trebuchet unit to it to give it a try. Or, better yet, create your own tech tree. That's all possible, thanks to the open nature of this game.
