Is it possible to change the default spell casting animations to something else via a script?

I was thinking that maybe Debug.SendAnimationEvent(akCaster, "animation") would work, but I haven't tried it because I only know how to use OnEffectStart() which would mean the default animation would have already played to begin with instead of replacing it(assuming it would work).

Any ideas?

Don't use SendAnimationEvent. To point you in a possible direction, look at all the master level skills (dead thrall, etc). The magic effects have keywords such as "RitualSpellEffect" or something similar which give them an alternative casting animation. I'd find those keywords and see what uses them.