I think this idea has a lot of difficulties.
First, overly restrictive codes of conduct, which the paladin already suffers from, might proliferate. There's also the possibility of unclear codes of conduct. (How many times can a paladin lie before they fall? Does a lie of omission count? Can a paladin lie to an enemy? Can you lie because it's a better idea than punching someone's face in? Etc.) Palladium (which from a mechanical PoV is a terrible system) has much better alignment rules than D&D and Pathfinder. The LG-equivalent looks a lot like the paladin CoC. There's 11 precepts, and I don't think you must follow every last one of them. If you followed 9 or 10 of them, and left out the problematic "don't lie" problem, you shouldn't lost your alignment.
Even a reasonable code can have issues. Back in 2e's Forgotten Realms, clerics of a particular deity (I think this was Mielikki, popular because she supported good-aligned druids, which were otherwise not allowed, but I don't have the source document anymore) had to plant a tree every "tenday". Seems reasonable for a nature worshiper. This is perhaps not so reasonable if you're taking a month-long journey by ship though, or traveling through a desert, or visiting certain planes, or traveling in the Underdark, etc.
Second, how restrictive a code can a cleric of a chaotic deity need to follow?
Third, what about "atheist" clerics? (As in those that follow an ideal, or the like?)
I think the druid's "code of conduct" is a good example. It's got things that don't make logical sense in it, but it's pretty clear. For instance, you can't wear metal armor, period. Maybe some people think that's stupid, but a player isn't going to "accidentally" wear metal armor, so you don't find fallen druids all over the place.
