Have a browse through this:
NWN Lexicon (Sort of like a Wikipedia for the nuts and bolts of NWN)
NWN Lexicon Function List
NWN Lexicon Constants List
I also suggest having a look at Lilac Soul's script generator- not as a crutch but as a learning tool for the more basic scripts. Don't go near it if you're looking for something more complex, it's only useful for fairly simple scripts.
Disco's right that a trigger in that fashion would be pretty much the best and most reliable way to handle it. Just be warned- once you place that trigger it's going to be all kinds of hell to do anything in that area afterwards unless you turn display of triggers off. I've placed areawide triggers before and you have this annoying habit of clicking on them and moving them (thus messing up their geometry, which the 'undo' function can't fix) and forcing yourself to redraw them.
But yeah, the trigger will only need to run once in that case. Or you could try dropping it in the OnModuleLoad script and skip the trigger entirely, though I personally haven't tried making said script apply effects like that on loading the module. Either way the idea is to only make it run once since that's all you should need.
I'm not the best of scripters but I know a couple people who are practically legendary. I won't name names since they probably wouldn't like it, but I'll have a chat with the ones I know play here and see if they can give you some pointers. Disco's probably as qualified as they are but he's got this whole module thing to run. Pesky responsibilities.