Financial companies always had significant amounts of off-balance sheet assets and liabilities, and securization only increased the amounts of assets held in off-balance sheets. Secure computation can be used to create autonomous trusts that manage said off-balance sheets, and then aggregate the balance sheets only when required.
On one side, all the details from the on-balance sheet are specified:
On the other side, the details from the off-balance sheet are specified:
Both parties securely compute the sum of the off-balance and on-balance sheets to obtain the actual balance sheet.
The preceding is just a simplified example for illustrative purposes. In the real world, you will have to provide parameters for your specific situation and use more complex formulas.