Depending on what your overall goal ist,
you can just take the player position when he places the siege weapon (pr during interaction) to get a rough estimate.
For more precision you can add some siege-depending offsets and such with some trial and error.
I'm trying to be fairly precise in the siege location. There are some locations in Cyrodiil where placement is tricky; for example, where the ground is only level in small patches. I want to record a known-good location and put a 3D marker on the ground (like the HarvestMap markers) to help me find it again. The 3D markers are easy, but I can't get an xyz for the siege engine.
I like the idea of using the player location plus an offset as a workaround, but since players can choose the placement of the engine, I'm not sure a static offset distance from the player location would work.