Shwankie and I are taking our geeky pastimes to a whole new level: Con Costumes.
I have never been to a Con. That's my caveat. I have only Halloween experience, and it's my understanding that con costuming is a completely different level of costuming from the usual Halloween stuff.
I actually dressed up as
Trinity and enjoyed it, but that was a long time ago and I don't know if I'd do it again now. The hardest thing about that costume was getting the fake gun.
Before that, and before I had a digital camera, I did Jadis from "The Lion, The Witch and The Wardrobe". It could be steam engine time for a good Jadis costume, because when I did it I had to tell everyone who I was, but now she'd probably be recognizable, since the movie.
A friend of mine has done
Lara Croft, a costume which is both easy and recognizable.
I like the idea of Zoe from Firefly, mentioned upstream, but she was my favorite character, so maybe I'm biased and again, that's girl with guns.
There's plenty of strong, distinctive women characters to choose from in the Buffyverse, or in something like "Alias" but those might not be SF enough for you. (Personally I'd go as Anya in a bunny suit if I was going to dress as someone from Buffy).
Your examples (so far) have been visual media only, but if you go to books there's tons to choose from. As TAD points out, the Vorkosigan universe is wealthy in distinctive women from Cordelia to a Cetagandan noblewoman to Elli Quinn and the aforementioned Taura. The Vorkosigan universe also has strict and detailed descriptions of dress (especially uniforms), which could be helpful when it comes to making a costume. There's obviously plenty of Tolkien openings (but again fantasy, not sf). I'd probably choose someone like
Jenny Casey from Elizabeth Bear's novels to dress up as for a con because she's a strong character, old, and distinctive looking with her artificial limb and I'd probably be the only one dressed as her.
Shwankie could go old school too, as the original Bionic Woman, frex, or someone like Rydra Wong from Delany's
Babel-17.
Anime and comics can give you ideas, too.
Ultimately she's going to want to choose someone from a fandom that she has really loved a lot because costuming is a lot of work and just not worth it for something you aren't all jazzed about.
There's also no rule against crossdressing, I don't think. She doesn't have to go as a girl.