So anything that requires changing all the references to the new one is a non-starter. I want something that lets me fix it in-place without going and hacking

So anything that requires changing all the references to the new one is a non-starter. I want something that lets me fix it in-place without going and hacking their source directly. If you could do something like "monkey patch this, but only for calling code inside my private namespace" that would probably get you 90% of the way there.