Adding Watermark to Screenshots in Flutter App for the native

Question

am working on a Flutter app and I need to implement a feature where a watermark is automatically added to screenshots taken by the user from within my app. This functionality should work on both Android and iOS devices. While I understand how to handle screenshots within the app using packages like screenshot, my requirement is to add a watermark specifically to screenshots taken using the default operating system functionality (e.g., power button + volume down on Android, or power button + home button on iOS).

Is there a way to intercept these system-level screenshot events and programmatically add a watermark to the resulting images before they are saved to the device's gallery? I would prefer a solution that doesn't involve ejecting from Flutter and writing platform-specific code in Java/Kotlin for Android or Objective-C/Swift for iOS.