I have done this in my game and its prety cool.
I made a set of clothes (child sized to fit my 1/2ling monk character) that were treasure intombed with a child in his sarc, sarcoph.....coffin.
They were made of a black silk type fabric, which had fine mythril strands woven into the cloth. Since it was for a child it was only +1 armor bonus to AC, (yes I say it's an armor bonus) and a DR of 1/-. Also it was black and used by the young royal to hide in the shadows and spy on his folks and others in the castle. So they add a +5 to a hide check in darkness only.
Now, my players are kind of savy so I had to mention that a few items like this had been found before but the secret to making them is as lost as the civilization that made them. This keeps this type item in my control. I gave this to my monk player to balance out his share of the treasures in the group. I try to keep it fair that way.
It adds a little bit of mystery to the ancient civilization and a unique item for the character. I'd say it'd be fine to let them discover the secret to making it eventually. It makes a nice set of protection for a wizard/sorcerer or rogue character. Like someoen said teh clothes themselves have no bonus so with the +5 limit on it the item is not all that lopsided.
Anyway, that's my two coppers......