1. I agree with Ijen. Or more specific: to damage a siege engine I would say it takes any normal weapon; the time spent damaging the siege engine is a big factor for the time needed to repair it. To damage it beyond repair you will have to burn it, which needs special equipment and preparation.
2. Since siege engines are part of the rules of sieges, they share some rules with palisades: every siege engine must have one owner who is responsible for it, must show it to the orga, and is allowed to handle/repair it (others may help) (and the part about seige engines adds that only the owner may fire it, too). If you bring it, and if it is approved, you can repair it. There is no skill "build/repair wall" either. And I agree with you, this is a big advantage that you buy with OT money. Like wearing plate armor.
3. You may use anything that is not (OT/IRL) possibly dangerous to a walking person. So I would allow you to use chalk signs (do not use anything that is bad for the environment or hard to get rid of; to use small ammounts of straw would be fine, too). You will have to clean it up, too. But this brings to mind the same question you have to answer for point 4:
4. There is no rule that I know of that tells you how to aim your weapon. There IS a rule, the first and most important rule, what must NOT happen upon impact: "safety for you and your target is always first priority". So while it would be cool to watch a cubic meter of foam material launched over 100m on an advancing enemy, upon impact this would cause a safety hazard if you hit the head (extremly likely in that case). Even more if your target did not see the projectile coming. So range and size of the projectile are limited. There is no exact rule except you have to get clearance for the siege engine, which you might not get if it is too large.
5. siege engines that use chemicals as PROPELLANT are forbidden, same as engines that launch something out of a tube, or with a hard core. So no cannons. Also no fluids as projectile (no waterbombs). But you are allowed to use oil/lubricant within sensible limits to ensure the proper working of your siege engine.
Ultimately: To give clearance for siege engines is a privilege of the heads (though as far as I know, even of those only Fabs and Benny do this). Anything you can get past them, is OK. Everything else, not
. I hope what I wrote helps you anticipating what they will accept and what not.
If you are looking for me at the CoM, search for the enemy siege train
. "My" artillery guys are looking forward to nuking it out with you.