Sounds fun! Make sure to share any war stories this campaign creates!
There are a few listed in the DMG, pg 99-100 (Catapults, Ballistae, even rams and siege towers). The Ballista is just a Huge heavy crossbow, so I bet other weapons could get the same kind of treatment to make them bigger (A catapult launching Huge size alchemists fire anyone?).
There are some in the Arms & Equipment guide right? I know they have rules for vehicle movement and such, so I imagine they have siege equipment.
Edit: I did a search on rpgNow and found some interesting stuff: http://www.rpgnow.com/product_info.p...ers=100_0_1100
This gives information about an underground army of dwarves in a cheap downloadable PDF using the D&D compatible OGL liscense. If you look at the bottom, of this summary you can see a bundle of a group of items called "Feast of crows," which is a system of abstracting large battles. The same publisher also offers a pdf with info about a goblin army. Very interesting.