Actually, don't we already have a slot system? We already have a limit of one platemail-like armor per body location, the locations that can equip armor are set to specific tokens in the body raws, and having unusual body types are not particularly supported. (I.E. serpentmen can wear shoes.)
As the example probably implies, however, this isn't necessarily the best sort of setup, either.
A more robust system than the one we have is sensible, and it would probably work best by having some sort of restricted mobility that is cumulative (you can layer plain shirts much more easily than platemail armor, but put on twenty shirts, and see how well you can flex around). This would require some sort of way for you to see how many penalties you were accruing on your units, however, and possibly having a way to control how restricted they are allowed to be with their clothing, or else more sanity checks with the way that dwarves dress themselves.
One that would use a token system that doesn't use the gamey looking for [STANCE] token method, and instead is built for specific types of bodyparts with a variable token like [BODYPART_CLASS:Foot] where you could define odd forms of clothing for oddly shaped creatures, like the animalmen who might want something odd like wing warmers to wear.
Also, if we're restricting motion in certain bodyparts, it might be helpful to deal with the odd properties of things like coverage from multiple pieces of armor overlapping coverage in intermediary body parts.
Basically, I think Toady is going to revist this at some point and make it a more sensible system, and it's just something he's putting off for a while.
And you seem to be forgetting that armour in this game is a cookie cutter style armour, with each armour being predetermined in the raws. The armour the game bases itself off of is armour that is meant to be worn in 1 layer, two at most. One hauberk could weight up to 50kg, good luck wearing more than one as it's designed for the user to only wear one. Sure, you could wear a hauberk and a chest plate for a little extra protection but you would overheat and not be able to move very far very fast.
I believe that Toady has referred to having armor that only one character can wear as being a "nightmare situation" in the past.
(It's locked, so I can't direct quote, but here's the post:
http://www.bay12forums.com/smf/index.php?topic=30026.msg994813#msg994813 )
Will the tall dwarves be able to equip human-sized equipment in the upcoming version? Or is it eons away?
I think it's a little farther away -- the flipside is having to size clothing for different dwarves, and that is a nightmare. Of course, you could go one way and not the other, but that starts to get kind of ugly itself.