Sweet! That sounds really cool. You should definitely bring it.
There are, sadly, no official rules on what legalizes siege weapons in Dagorhir. However, the current crossbow rules are:
4.5.3.3. Crossbows must have a maximum draw weight of 35 pounds or less at 12 inches power stroke. Power stroke is defined as the distance the string moves from its rest position to its cocked position.
Also, any bolts you make must not weight more than 1.5 lbs. If you're adamant about making some mini ballista bolts just let me know. XD