Well, to do that it would have to include a site sucking tool like iCab has had for a long time, but then you get issues with it downloading content you don't need or not going down enough levels in the hierarchy etc. I suspect getting a tool that downloads the content just for the page you are viewing in the correct folder hierarchy could be quite difficult to code. I also suspect that it is something we are very unlikely to see in OmniWeb 5.x ;)