Oracle Payments - AP Payment Validations

Hi, wondering if anyone encountered below scenario and can share their experiences.

Our client has a custom payment format program that does lot of validations depending on the country and the type of payment that is being done. While I understand and have explored the option of utilizing standard and custom validations that can be set at payment method and formats I see a limitation in Oracle Payments that we cannot put our own logic to define new validations. The custom validations provided by Oracle Payments are limited to the options in setting them up and won't give us what we're looking for.