After testing on various mobs, I've concluded that Worms are just an exception to the rule (but then again they already are even without taking the recast timing into account, since they don't cast while their target is in range.)
Attached is the .txt file where I was writing down the spells used, cast times, and time differences of the mobs I was examining, 'til I decided writing it down was taking way too long. Basically, you'll find a little bit of data on Land Worms, the already posted Ahriman data, and a much lengthier piece of data from a slightly lower level ahriman mob, a Morbid Eye. Figured I'd post it if anyone was curious.
Also, here's a .zip with all the screenshots I took, if anyone wants to examine them personally. Every now and then you'll find an extra accidental /clock or something of the sort. I usually marked them in /echo with some sort of short phrase along the lines of "null".
http://files.filefront.com/Mob+Casti.../fileinfo.html
Edit: By the way Rena, I appreciate the effort. I WAS going to be FFXI-less for the whole week but plans changed and I'm staying over at a friend's apartment, who actually has functional enough internet to allow me to play. I would be interested in seeing a small sample from Imps though.