Does she give you hairjobs? I think the first step would be to get a hairjob, then you could ask her, as long as she's turning you on so much, can I please cum in your hair?

If you think it is easier to get forgiveness than permission, cum in her hair during a hairjob and tell her it felt so good you couldn't control yourself.

If she's not giving you hairjobs yet, tell her how amazing her hair feels against your face or chest or thighs, and see if she figures out on her own that it would probably also feel great around your cock.

Good luck, and let us know what happens!