Anything using the head slot will hide the hair. The only add-ons for the head I've seen, that preserve the hair, use the tail slot, and therefore are unusable by Khajiits and Argonians.
In theory, any wearable can be nominally assigned to any slot, but it will remove any other item in that slot (e.g. hair) when worn. The mesh itself is weighted to the same skeleton as the body, and can actually be worn anywhere, not just covering the designated slot. So you could have a tiara made as part of a dress (body slots) and not lose the hair, but it would always be taken on/off with the dress. If you then put on a hood or helmet, it would only displace the hair, not the tiara, so that approach also has problems. Another option is the amulet slot, if it's something to wear with armor, but then the armor must be marked as not hiding the amulet. Complicated, isn't it?
Yeah, I just asked about that head and hair slot at Construction Set area
I tried to use ring slot (ring is not quite visible in normal gameplay, if at all) and it works quite well, if no equipments has Hide Rings attribute
Anyway, what about that hair ornament mod? It can be at any slot except of course body, hands, feet, head and hair. Currently I'm using re-textured Moon Lover's hair ornament, occupying left ring, and while I'm quite satisfied by it, I certainly should use something better
OOT: Anybody ever tried Quiver slot? Might make a great slot for non-archers
EDIT: another thing: I just saw http://www.tesnexus.com/downloads/file.php?id=31084, and I find the character's hair very pretty. Anyone knows what hair is it and where can I find it?