Edit::embarrass I didn't realize you meant 3.x rules for piecemeal armor...I hope my post didn't come across as snarky.
The whole thing has not been updated for ages, but the piecemeal rules are still pretty much what I'll keep them to be, I guess. A bit more bookkeeping, of course, but allows for more flavor, and brings back this interesting variant of the Dark Sun system.
If you go see, and your interested, I'd love to get some comments on it.
For instance, you wear a full suit of scale mail. This usually includes gauntlets, and covers all 4 limbs and the torso, provides a +4 armor bonus, a max dex of +3, armor check penalty of -4, etc etc. If you wear just the torso section, this gives you half of the protection of the full suit, so +2 armor bonus, +6 max dex bonus, and an armor check penalty of -2. As for movement penalties, unless the wearer has more than half of the suit on, you can reduce the armor speed penalty by one category (ie medium to light). Arcane spell failure is divided the same way as everything else, and of course, you can't enchant peicemeal armor unless its a full suit so you don't need to worry about this aspect. Likewise, wearing only one arm of scale would confer 1/8 of the protection, while 2 would provide 1/4 of the protection. This was roughly how it was done back in the 2nd edition boxed set so its a quick and easy solution.
Helmets don't really do much according to the 3.5 PHB so its not essential to wear them with a suit, except to look cool.
I find your suggestion a bit funny considering its compared to mine, since that's essentially, if not exactly, what I did.
The thing is, I gave each armor part a fraction of its own, since its eaiser to keep track if you wear shell leg greaves, scale arm bands and a bronze breastplate. Other modifiers, such as max dex, check penalty and arcane failure were calculated the same way, but with some variation to account for the differences between the impacts of each armor piece on your movement.
Also, when wearing half or less of an armor type (such as just the guards, or just the breastplate) your armor is effectively considered to be one level lighter, to a minimum of light, which allows characters with light armor proficiencies to still get to wear some armor (this is especially flavorable for gladiators, I think).
And also, note that simply dividing the armor bonus didn't always work. A chain shirt, for example, is just that, a chainmail breastplate if you will. Now, if you could purchase and wear just the chainmail breastplate, with half the bonus, you'd end up with less AC than the chain shirt! So I adjusted a bit, making some full suits to give the proper AC bonus (padded, which is only effective when everywhere, chainmail, which would be the unseperable hauberk, and full plate, which offers the extra AC point because it fits so well together). The breastplate offered the same problem when compared to the full plate, with a bigger bonus on its own than half the bonus of a full plate, and even more than the breastplate of a half-plate suit. That needed some tweaking as well.
Note that put a lot of thought into the divergences to have it justified by the pictures of the PHB. In the book, breastplates come with leg greaves, explaining the extra AC point. Chain shirts also come with a helmet, so its not that far off.
And about the helmet, that's a personal preference. The way its done, a helmet could never grant an extra AC point with a full suit of regular D&D armor. However, it might give you that point with piecemeal armor. So its an incentive to wear one, which I think is really something that's neglected in D&D. A guy in full plate without his helm wouldn't last too long in melee against the helmed one...
Anyways, I'll probably re-edit the document, but I'm pretty sure all I just said, and what you suggested, is included already.
Helmets don't really do much according to the 3.5 PHB so its not essential to wear them with a suit, except to look cool.
That is unless you incorporate something akin to the book Critical Hits, Torn Asunder by Bastion Press (yes, another shameless plug brought to you by unfriendly to his own neighborhood Mach). A hit location system coupled with piecemail armor just seems to go hand in hand to me.