Mercury Savings & Loan has hired Montgomery Securities to help it deal with inquiries it has been receiving about the possible sale of part or all of the S&L, which has $2.3-billion in assets. The investment banking firm also will advise the Huntington Beach-based S&L on business and regulatory matters. Industry sources said the action indicates the S&L is up for sale. However, such an indication would be "improper," said William A. Shane, Mercury's president who last week announced that he is resigning soon. While several companies have inquired about buying the S&L, it is not negotiating with anyone now, said Leonard Shane, chairman of the 23-year-old S&L and William Shane's father.