Basically, they would be people who would need to attend all board meetings and who would have a specific obligation to immediately report any activity that is either illegal or a possible threat to the financial system as a whole.
They could be company insiders who are specifically charged with this role, with rewards for doing it well and penalties for doing it badly. Alternatively, they could be civil servants who are knowledgeable about the firm’s line of work.
Arguably, this would just lead to nefarious activities being orchestrated in venues other than board meetings. Even if some of that happens, it could still be useful. At the very least, it would obligate nefarious board members intent on breaking the law to arrange ways to trick the designated whistleblower, which would interfere with some kinds of bad behaviour. Also, having a designated whistleblower constantly present would be a reminder to others that you are allowed to point out unethical behaviour when it is being practiced by your employer.