You can download DLTCEP right here, while there is a support forum found right here.
Second of all, this should be possible in BG2: ToB, although I am not sure about BG1: ToTSC.
Now on with the tutorial:
This tutorial will cover on making mages recieve a saving throw bonus against specific spells. This will require you to edit every single invocation spell in the game that allows a saving throw. Let's start with fireball. I'll explain everything step-by-step.
1.) Make an external effect that grants a saving throw bonus vs. spells of '1'. The duration is irrelevant (as the spell's duration overrides the external effect's duration), as well as the resisttype, but the target should be set to "self" (again, the spell's targettype overrides the external effect's target type, although most effects use "self" as the targettype no matter what).
2.) Save your external effect. We'll use "X.eff" for the sake of this tutorial.
3.) Open up the spell "SPWI304.spl" using DLTCEP's spell editor
4.) Go to the extension headers section.
5.) Create a new effect of the spell with the opcode number of "177; Use Effect File".
6.) VERY IMPORTANT: The effect should be the first thing in the spell's extension header, when it comes to the order in which the effects fire off. You will see why I am doing this in the future steps of the tutorial.
7.) The effect should have a duration type of 'duration' (or '0') with the duration value set to zero. The reason it is zero, not one is that having a duration of zero will make this effect affect those effect files that are only in this spell. Having a duration of one might affect other spells that affect the target right after this spell affects that same target.
8.) The resistype should be set to '1' ('dispellable/not bypass resistance').
9.) The targettype is, of course set to 'pre-target'.
10.) Remember that the power value should be set to 3 because this is a level 3 spell. Spells such as Minor Globe protect against effects using the appropriate power value, not the spell level. That is why dispel magic works against the minor globe while being a level 3 spell.
11.) Now in the resource key, type in the filename of the external effect you just saved. In the tutorial's case it is "X".
12.) Now fill out the parameters so the effects only apply to mages. This will allow mages to recieve a saving throw bonus before the rest of the effects in the spell fire off on the character that include effects requiring saving throws.
This is just an example on the numberous ways you can play with this method for other results (ie. saving throws against specific spells for specific kits). Let's hope this will ring a bell for any of those people who thought such things are hardcoded.
Unless you are accastumed/willing to edit dozens of spells for each such effect, it is a very perilious task that is usually best left to mods that revamp half of the game rules.
FYI: My next such tutorial will cover on making specific spells that grant saving throws against specific spells.
-Galactygon
