Is it permissible to open a commercial register for a Saudi national to be used by a relative of his who is of Yemeni nationality, knowing that this is considered prohibited cover-up (tastur) in Saudi Arabia?

If the state prohibits granting a commercial register to anyone other than the person in whose name it is, for an apparent public interest, then it is not permissible to violate that, whether for a fee or without a fee. It is also not permissible to take a fee for the commercial register from someone who uses it through rental, because the state prohibits that.