For high risk scenarios like this a private channel for the top 20 witnesses, plus those witnesses close to the top 20, is required for security reasons. Such a channel exists and the only requirement for entry is one's position in the witness rankings. If a witness is in the top 20 they are in that slack. If there is a chance they may enter the top 20 (e.g. if they are close) they should be in that slack and if they are not, they should contact me at [email protected].