Check out this amazing video that was recently spotlighted at the Minecraft Forums, where a group figured out how to turn a ghast into a home defense weapon!
The coolest part is that it can be done in survival mode... but you'll have to be precise and careful. The video includes links to other videos showing how to capture and bring the ghast from the Nether (hint: minecarts and fishing rods).
So, if the server admin keeps denying your request to add a cannon mod, use what's available and make your own cannon! Imagine the look on your opponents' faces when they see a ghast float to the top of the tower and rain fiery death in their direction!