Android webview with Javascript: is there a way to inject the interface without modifying the frontend html code?

Android webview with Javascript: is there a way to inject the interface
without modifying the frontend html code?

I'm working on a wrapper for a web application that will run on android
and on ios. I have created a simple native interface as so:
in my WebView I have included:
addJavascriptInterface(new MyNativeJsIface(mContext),
NATIVE_IFACE_JSNAME);
in the init() method.
I proceeded to implement the interface and added the following line to a
test html I am using for development:
var myNativeCallIface = window.nativeIface;
and then I am able to call my methods that I defined in my api
successfully from the html like so:
alert("response from native:" + nativeIface.method1() );
So far so good right?
Wrong...
The problem I'm facing is that the code I'm not going to be able to load
this code into the html of the web application itself, and I therefore
require a way to provide that interface to the developers of the web
application without actually changing their code.
What I'd like to do is to make this code available so that when the
application is running on android it will load that code and when it's
running on ios it will load the ios implementation and I won't need to
write a wrapper around the methods to check the user agent and act
accordingly.
Is this possible? Am I going about this all wrong?