Rav Adiel Asyag of Kollel Eretz Chemda issued a psak by which under certain circumstances a woman who immersed in the mikva with "rastot", or dreadlocls, in her hair, the immersion could be considered valid.

Jack Sparrow can now go to the mikva!

Normally dreadlocks would be considered a chatzitza and would invalidate the tevila. The braids are tied very tightly and that is a problem. However, Rav Asyag says, that if the rabbi knows that if he says no she simply won't go to the mikva, or in a situation such as a baalas teshuva that might reconsider because of such a problem, there is room for leniency and to allow her to immerse even with her dreadlocks intact.

Rav Asyag qualifies it by saying that she must clean the braids very thoroughly and ensure nothing is stuck inside them. As well, such a woman should stay under the water for a bit longer than normal to give the water more of an opportunity to get though the tight braids.
sources: Kipa and Srugim