I remember having this question on my mind for about a week or so now, I forgot who asked me to ask it though. Anyway, the shaykh, Dr. Muhammad 'Umar Baazmool, responded:
"The divorced woman from the first or second divorce can request a khul', as she is considered his wife even during the 'iddah, and there is nothing preventing her from seeking a khul' during this time since she is his wife. As for the woman who has been irrevocably divorced (the third time), then she may not request a khul', since she is not considered his wife during the 'iddah. This is what seems correct to me, and Allaah knows best."
Moosaa Richardson
******************** سبحانك اللهم وبحمدك أشهد أن لا إله إلا أنت أستغفرك وأتوب إليك