That's good news, and thanks for the test script. This reflects the type of substitutions I need:

#!/usr/bin/node

var vec = { x: 1, y: 2, z:3 };
var obj = { id: vec, key: 'Wrong key!' };

function do_meth( obj ) {
    return obj.id.y;
}

function do_windex( obj, key ) {
    return obj[key].y;
}

var key = 'id';

console.log( do_meth( obj ) );
console.log( do_windex( obj, key ) );
[download]

Where do_meth() is how it is, and do_windex() is how I need it to be. And, it works for my big example.

At least mostly. There is still something that isn't quite right; some small difference I haven't tied down and parameterised properly.