Combat is only loosely based on 2da's - generally most of the rules (at least in the case of NWN) were fairly hardcoded. For spells and the like most of the values were 2da driven.

The way the system works is that the animations associated with a command aren't exposed - it's a hardcoded call in the engine. To be able to change a projectile for a spell on the fly you'd have to either:

a) Be able to modify the 2da/data on the fly as that's where the data is retrieved from.