Never used it, and I am certainly no expert on CEP, but:
I believe code 900 is a hard-coded effect for the magic missile. Notice the column used for the colored projectile is actually an impact column. That would be why the projectile is sticking to the target, rather than making the correct path. From my point of view, this looks like somebody's wishful thinking, but I could be totally wrong. I do bet that an engine hack could be, or has already been created (that I don't know about) which could make use of that data and change the color elsewhere, like NWNCX does for visual effect beams.
You'd probably be better off cloning a fireball-like and using the ProjModel column in spells.2da. You'd either need to make a very complex single missile VFX, or fire the same spell multiple times, that is if you needed multiple missiles.
You can probably fire multiple spells instantaneously by using invisible objects around the caster and making fake spell cast calls from each of them at once. In that case, you'd need to either have them on hand in the area, or have the spell script create them every time. But at least that way, you could give the projectile any model you wanted, and also change the path type. Calculate damage per hit in your script as if all the missiles came from the same caster.
The odd thing is that to fire all the missiles at once, you'd have to have one spell (without a projectile setting) which called another fake spell to fire all the missiles because of how the projectile action works with impact scripts.
That may be exactly why they hardcoded it, because nobody wanted to work out a better system when basically only one spell in D&D worked that way.
