If it was me, I'd find out how to contact her, and simply sasy, "I'm a guy, I was completely oblivious, and I'm very sorry". Might help her move on, and it's a 'good guy' thing to do.
<waits for others to say, "this is a bad idea"...>
Nah, I don't think so. We have mutual friends on Facebook but I have not tried to friend her. This was over 10 years ago. I'm married with two kids and I'm pretty sure she is married with a child now as well. I think I'm just going to leave this one alone. Like I said, I don't even really know her. I went to high school with her and gave her and her friends a ride one night. That's pretty much it. I didn't tell this story as a way to ask advice, I just thought it was food for thought pertaining to the topic. I don't feel particularly guilty over the situation as I never mistreated her or badmouthed her. I was always cordial with her when she greeted me at school. Just something to think about. She is obviously over it by now.
Yeah, "no thank you" would be less an ego-blow than, "Well, let me tell you why I think you're not dating material. I'll give time to get a pen..." 