If you know a  chick was attracted to you,  and you guys never worked out or got in a relationship  for reasons out of your control (bad timing she just got out of a relationship, or  she caught you talking to another girl and assumed you weren't interested etc)   I don't see why it would be a blow to your ego to  remain  friends with her while she continues to date  other guys.  Why take it personally if she liked you at some point? 
I think guys blow the friend zone thing  way  out of proportion, I've banged  a lot of chicks who I thought I was in the friend zone with. When a girl cheats on her boyfriend who do you think she runs to? One of her friends lol.  If a girl has ever gotten physical with you and uses the word friend with you that's how she convinces herself she's not a bad person for leading you on, while she's really keeping you around as a potential side guy. Give it a few months and wait for that midnight phone call a few months down the line lol