This is a very interesting thread indeed, well the clear choice of a new siege unit is a ballista, but since isengard already got such a unit, it would be better to look for a new unit, i remember the high/blood elves from warcraft using arcane golems (
http://wowwiki.wikia.com/wiki/Arcane_golem) as their siege units, they actually use them against troops as well, but i am trying to suggest is making perhaps rock golems summonable by lightbringers or mages, i would say they will be like a mini mountain giant ( wich appeared in the hobbit of course ), they should be very slow ( slower than ents ) but much resilient to arrows, fire and magic damage, also if the mage that summoned the rock golem is killed, then the summoned creature dies as well ( i guess you can use that angmar summoning master mechanics, when he dies all his battalion dies with him ), i know it isnt very lore accurate, but since elves ( specially the noldor ), were in contact with great magic of controlling the aspects of nature, i mean the three elven rings are synced to nature elements, this would permit the light bringer to do damage to buildings in a new and different way, or if you arent satisfied with the lightbringres, you can make the rock golems summonable from their building, i mean whenever you summon one of them, we see the animation of a light bringer raising his staff and summoning the creature.
I just said what i think of, so please be nice in your replies