I was thinking about modding a few aspects of the Radiant Word Wall quest (WICastMagic04), and I've been trying to learn exactly how it works. However, because the quest is both Radiant and a response to a World Interaction, it's harder to decipher it linearly in the Creation Kit than a quest with set values. I'm specifically having a hard time finding the trigger that actually causes the Courier to pay you a visit. If someone could point that out to me, I'd appreciate it.

Bumping for words of wisdom.

If I recall correctly, it triggers off of the Cast Magic Event:
http://www.creationkit.com/Cast_Magic_Event