> - rendering to a window that isn't owned by the UI being rendered or created by the framework

I once used a native library I installed with iOS' native package manager, Cocoapods. I rendered a React Native view inside the native view, which was contained inside a react native view. The only limitation is that the two react native views used separate bundlers i.e. the two react native views can only communicate through the native platform, they couldn't communicate to each other via JavaScript.