Is it permissible to count the amount paid on behalf of the insolvent debtor as Zakat, if the debtor will not repay his debt?

As long as you have guaranteed the borrower to the lender, you are obligated to repay the debt on their behalf. What you pay cannot be counted as zakat, because you are fulfilling a debt that is incumbent upon you. Zakat is the transfer of ownership of a portion of wealth to a poor person with an intention coinciding with the act of giving. Furthermore, the payment was not made to one of the prescribed recipients of zakat.