Is there any way to, when handling an activemagiceffect event, to obtain the magnitude and duration values? I'd like to do some script-based processing based on those values.

Or am I just going to have to kludge it with global variables?

It doesn't look like it, judging from the documentation. I'm trying to do something similar, what I've ended up doing is creating a script type effect that has a bunch of properties, and I use the properties in place of the magnitude/area/duration fields. The drawback is that you need to create a new effect for every magnitude/duration/whatever you plan to use.

You would think so, wouldn't you ... But I'm struggling to find out how too ...

You can trap that an ActiveMagicEffect has started: http://www.creationkit.com/OnEffectStart_-_ActiveMagicEffect

There is also an event for when it finishes, but that is noted as somewhat unreliable

I'll keep looking and edit this if I find a way ... It is, after all, a property of a spell ... And so it should be retrievable at runtime ... But it seems not to be (documented, at least)