Chahar-aina and dastana

[Kaiwen]

Chahar-aina is an armor that can be explicitly worn with certain other armors. I assume it shares the body/armor slot with your chain shirt.
Dastana are an armor that is decribed as bracers that can be explicitly worn with certain other armors. Do they take your bracer slot since they're described that way, or do they also share your armor slot since they're armor?

Both appear in the armor section of Oriental Adventures. Relevant text:
This “four mirror armor” can be worn over certain other types of armor to provide an additional armor bonus that stacks with both the foundation armor and any shield worn.
These large metal bracers can be worn in addition to some other types of armor to provide an additional armor bonus that stacks with both the foundation armor and any shield worn.

[Maat Mons]

I personally take the view that it is the magical enhancement that determines item slot, not the physical item.

So for example, if you add a +1 enhancement bonus to a gauntlet, I don't think you need to use up your hands slot to benefit from the +1 to attack rolls, because magic weapons don't require a body slot.

[Kelb_Panthera]

Cahar-aina plainly occupies the same slot as the base armor it's stacked onto. I don't see how you could reasonably argue otherwise.

The OA version of dastana isn't the most recent one though. They were updated in The Arms and Equipment Guide to confer a shield bonus that stacks with other shield bonuses. As such, I tend to assume it's meant to occupy the same slot as the shield it stacks with, fitting over other gloves/ bracers you may be wearing and not interfering with their magic, if any.