I'm confused, we did this on US Tiger line file data in 2006, rendering it on a early equivalent of Google Maps. You have to decimate the polygons to get them to render but doing bounding boxes around postcodes is fairly reasonable.

I think the one I enjoyed the most was a zipcode with 38 separate geographical areas, and the other example was a street where one half was one zip code, and the other was a different one, but half way down the street they swapped sides. Representing that with non-overlapping polygons was impossible. So we just let them overlap and shrugged.