> the public facing URIs will never change.
The Internet is often changing and this is something to take into account. 
The W3C explains that clearly:

»As a general rule, resources on the web can and do change.»

Even »permanent» URIs may change ... There are several ways to consider 
such a flexibility. I don't know if this is what is meant by

> a perfect web world

Anyway, I do not know all the details around your problem and I cannot 
solve it, but my suggestion is to avoid assuming that anything at all will 
be permanent for ever. I thought that these two links could be useful for 
you to handle and / or rewrite relative or "semirelative" links: