Access Object3D loaded using OBJMTLLoader

The comment from @mrdoob led me to solving this myself. My variable obj3d was indeed out of scope when the jquery callback was triggered. I was able to resolve it by moving the variable declaration to the global scope.

Additionally, the reason I wasn't seeing the changes applied immediately in the three.js scene was because I needed to call the render() function.

$('#initial_y').change(function() {
    obj3d.position.y = $(this).val();
    render();
});

Hopefully this will help someone else. Thanks for pointing me in the right direction!