I'm the author of react-native-webview-bridge and what that project does is create a channel or bridge between WebView and ReactNative so WebView and React Native can communicate. Because of the nature of how WebView built in React Native for both iOS and Android, it makes very difficult to extends, specially Android. A lot of useful classes are defined as inner class which makes extending some how impossible.
What I'm proposing is if core team accept my request I'm willing to bring this feature to core as a PR.