Why can't you charm NPCs? That's a big hole in gameplay in ToEE. Why waste your time with diplomacy, intimidate, etc., when you can just mindrape that sucker and make him tell you what you need to know? That's how DnD 3.x works: Spells > everything else. So it's not like it would be against the rules or not in the sense of DnD, because it totally is in the sense of DnD to use spells to make others do thy bidding. Dear modders, can you include that in the game with your modpack?
It would probably require a sizeable rewrite of basically every NPC ... not sure we have the time/resources at this point.
I think re-writing NPCs is the least of your problems..... unrestricted charming (assuming both Rannos and Gremag succumb before they hack the party to bits) will certainly ruin the LG start - "I wouldn't bother with the Moathouse if I were you, all the valuable prisoners are taken to the ToEE - we don't know where that is but Rentsch/Wat in Nulb do - here I'll mark Nulb on your Map"
Actually, in real DND adventure, I doubt the charm effects on social activities. Charm spell can lead to attitude change: treat the target’s attitude as friendly. If you check the skill description, NPC attitude varies from Hostile, Unfriendly, Indifferent, Friendly and Helpful. Being friendly does not mean he/she will do whatever you told to. It may give you some bonus on your social skill check, but that's all. Even when you have diplomacy, bluff or intimate skills, the only thing matters is the words you choose and the points you take (while in-game mechanism leads to certain dialogs). You cannot simply throw a d20 and ask for help. And who would be "friendly" to a guy walking into your shop and try to cast spells on you?
I can just picture it ... Cast a spell at me, sure, here you go, you can have this crossbow bolt for free and keep it the rest of your life! Although, the rest of your life may only be moments.
as long as nobody prevents you from casting the spell that's exactly how it works. with a dc 20 diplomacy check the npc even goes from "friendly" to "helpful".