Thank you for reaching out to us, and for raising this important topic!
We do plan to add material to our site about sexual assault and its effects as relevant to niddah and mikveh. This is still in the earliest stages, however.
In the meanwhile, here are a few links to other resources:
Here is a poem in Hebrew written by Kineret Samuel-Pollack, who works extensively teaching the laws of niddah to those who have undergone sexual assault. Loosely translated:
Soferet Sta”m (Immersion)
Once a month
she immerses in clear ink
arises from within it like a feather
consents to continue to depict her life
to fill in the white mandala
seeks
to pour desire into obligation
waits for Torah
Here are two relevant pieces that have been published by The Eden Center:
Female Survivors of Sexual Assault and Their Encounter with Immersion in the Mikveh
On Sexual Assault and Mikveh Immersion (in Hebrew)
We hope you find these helpful. Please feel free to get back to us with any further questions.