Most likely scenarios:
A) If you tell her you know who she is before you see her, she will most likely not see you. Kind of creepy.
So, B) You just book and show up.
1) She doesn't recognize you = you do the deed. Then, you could tell her or not - she might freak out. Bear in mind, if you have any mutual friends, it's possible she could out you as well, so...
2) She does recognize you = you obviously pretend you didn't realize it was her from her photos. I think it's unlikely she goes ahead with it, but you never know.
Anyway, if you go for it, I'd probably have a solid plan B as I think it's likely you end up frustrated
*edited* I just read again and saw that you knew her fairly well - so she'll recognize you. As someone else said, your only bet is to pretend you didn't know it was her. Almost certain she wouldn't shag you though - I reckon the only possibility is if the 2 of you always had some chemistry and are now either both single or both attached. Same stakes on both sides then if exposed.