So does anyone know of a mod with a script that drains agility, speed, and stealth when you wear heavier armor, or drains stealth during daylight hours?

Not entirely sure, but I think The Lightning Mod adds so that you have a slight sneak penalty during daytime hours, and you lose your sneak when wielding a torch. I think there IS a mod that adds penalty to heavy armor too.

http://planetelderscrolls.gamespy.com/View.php?view=mods.detail&id=608 This is exactly what I was looking for, thank you.

If you haven't, check out the Empirical Morrowind list over at MythicMods - there's a list of thief/stealth mods there if you're looking for stuff like this.