Thank you for your patience. We have released custom domains for Azure AD B2C to public preview.
Documentation can be found here: https://docs.microsoft.com/en-us/azure/active-directory-b2c/custom-domain?pivots=b2c-custom-policy
We are planning to do this in the next 6 months, however it will only be possible through the Graph API.
An error occurred while saving the commentAnonymous commented
It is quit easy to design a change email flow with custom policies. You can overwrite the email field. It is not totally out of the box, but takes about one hour if you start with a starter pack and have a bit of experience.