I have this idea for a character, a female redguard ninja, but it strikes me that all the clothes that might have been good, mourners clothes or vampire robes or whatever, turn into dresses on women.

What clothes don't turn into dresses? Are there many?

I think the Redguard clothes (Hammerfell garb) stay the same, although it's been a while since my female character(s) have tried them on. You're right, most I have tried on change to dresses.

Well I wouldn't want my redguard to dress like a redguard, that would be weird, but thanks anyway.

This is definitely a problem, I hope in future games a dress is a dress and a pant-suit is a pant-suit.

Miners' clothes don't turn into women clothing. My Breton wears it all the time, it's suiting for her.

Hammerfell garb, roughspun tunic, ragged trousers, miner's clother, fine outfit (?). I think that's all, the rest are armors.

edit: I think Cicero's clothes too.

Miner clothes, they stay as they are.

The set of clothes soldiers carry. Every time I free a prisoner from three soldiers the three soldiers all have a set of clothes in their inventory, not anything exciting but they stay as trousers and t shirt on a female character.

Sorry I couldn't be more helpful. I was surprised as well how much most clothes change if you loot them from the opposite gender. All three of Nepos' bodyguards for example are wearing the same set of clothes but the way it looks on Uccaren's daughter is nothing like the way it looks in the two guys.

I never understood why clothing magically turns into dresses or skirts when worn by women, when they aren't dresses or skirts on male characters.

Well, there's always leather armor.

What if we opened it up for clothes-looking light armours? What could be ninjery?

Dark Brotherhood gear.

Vampire royal armor? I don't think Thieves Guild armor is ninja-like, but at least it has pants.

I guess she could be a brown ninja, although I'll feel like a kid with a "mighty machine man" figurine instead of the optimus prime one I really wanted.

I'd rather something I could enchant and rename as well.

Are you on PC or console?

Solders carry lootable "clothes" that are similar, if not the same as, minors clothes.

The Worn Shrouded Armor may be right for you. It can be enchanted and renamed. Its only catch, which may not be a concern for you, is that it can not be improved at a work bench.

http://elderscrolls.wikia.com/wiki/Worn_Shrouded_Armor

Pay attention to this bug though:

Grab the set on the book shelf at the bottom of the stairs leading to the training room in the Dawnstar Sanctuary. Don't even pick up the Worn pieces that are in the master bedroom. I've never had it happen with the set at the bottom of the stairs and had it happen every time with the set in the bedroom.

^ This. The female DB robes might work for you.

Because you need one male and female model anyway at least for upper body. of some bug lower class bottoms in Oblivion tend to be skirts on npc.

Some bugs are to funny to fix