My guess is someone at the brokerage house also doesn't understand who really owns the account. While it might look like the participant owns the account the reality is the trust does. As such the trustee can do what is being requested. It might help to try and get them to see that fact.
Been awhile since I've run into this but informing them that they are taking on direct fiduciary liability for ignoring the Plan Trustee's written direction often gets them moving in the right direction.