This is the exact distinction I'd like to make, in fact. The pension probably isn't managed by a union, it's managed by a board and management structure that wi

This is the exact distinction I'd like to make, in fact. The pension probably isn't managed by a union, it's managed by a board and management structure that will usually include both employees (who might be former employees, or members of one union, or a different union, or management) and representatives of the employer. The union most likely negotiates certain details of the pension in their contract, but actual management is not typically done by a union. Hence, they cannot easily use management of the pension fund (they don't manage it) against the employer, as the OP claims.