I think it's kinda weak to come up with a look-a-like story though. It's like 'hey I saw someone that looks like you!'...'O gee thanks, why don't you look her up instead.'
Get my point?
Just be real with her, 'Did we meet before? You look like a fun/cool/cute/hot/super sexy girl. You got anything going on besides X?'
You might wanna check this post
my-facebook-method-vt81168.html?highlight=
I've posted my result there also, it's the latest reply.
As for now, yes you're right she is atleast the slightest bit interested to reply. Maybe out of curiosity, but it's better than nothing right. Just interpretend every reply positive and you'll make it alot easier for yourself.
You could tease her about staying home, sure. Maybe try 'Wow what you been doing on your saturdaynight then? Playing world of warcraft all night? you nerd

' just be original with that.
Or you could come back with: 'O darn I think I've mistaken you for a hot girl, my bad! I'd still like to get to know you, since we're talking now anyways.

'
Let me know how you replied, goodluck!