I wonder if the companies that make a mk5-to-mk4 wiring loom could fabricate it in reverse for you? I can't recall a company name that does such a loom but they exist. This is tricky as there is a large number of fundamental changes between the two generations but I'd bet that it's not overly difficult to figure out, will just take a long while working out which bits of both looms you'll need. I'd imagine you'll have to use the 1.8T ECU and then work some coding magic on it to get it happy with all the features of the mk5 platform.
That, or run a stand alone engine management system and retain the stock ECU for everything else?