I searched for "seige" and found
this 3 page thread from a month and a half ago or so (
please don't resurrect it!).
Also, as has been suggested, there's
this entry in the Experimental Weapons FAQ.
The general consensus you'll see there and every other time it's come up (many, many times, hence the cool reception you're getting) is that:
a) There are no formal rules for it. Various reasonable suggestions have been made and used at times, but anything would have to be at least partially made up on the spot to suit the rules of the scenario and the safety concerns attached to that particular device.
b) They wouldn't be very useful anyway. Seige weapons were mostly designed for destroying fortifications and attacking large formations of tightly-formed troops at range. We don't do castles much (and we don't want them destroyed when we do), and we tend not to do large formations of tightly packed troops far away either.
c) Better safe than sorry. This sport has been going for over twenty years no in one incarnation or another, and has gone that whole time with no significant number of seige engines used. As a result, people are strongly disinclined to mess with a good thing, so any proposal will meet with heavy scrutiny. And some disdain, because it comes up over and over and over again, and always ends with the same conclusion.