How can I build my client app so that it can integrate with 3rd party services

At its simplest, you need to create an intent object for a given intent and then issue a startActivity call.

var intent = new Intent("http://webintents.org/share", "image/png", "data:image/png;base64,.....");
window.navigator.startActivity(intent);

or if you want to start an activity to edit multiple objects you can use the following:

var intent = new Intent("http://webintents.org/share", "image/png", 
    [
        "data:image/png;base64,.....", 
        "data:image/png;base64,.....", 
        "data:image/png;base64,....."
    ]);
window.navigator.startActivity(intent);