I am running the fluid dynamics search engine as a back up to my web directory. what i do is get the results from the directory first and if there are none, send then to the backup fdse without showing them the search error page. what i also did was to add a link on my templates offering "back up search" so that the user coudl be looking at the search results and then switch to the fdse results...it works pretty well and there are now very few peopel seeing the search fail page.

All i did was create a template for the fdse search engine so that it looked the same as the directory results page. They click on a link that is a search string using the <%query%> tag to go to fdse. the only problem is i cant get the user back onto the directory results because i dont know the string and what tag to use in fdse to search another location.
If it was possible to send a visitor from fdse to links 2...you could alter your templates to have a choice of "directory-web" and that could be the google effect you where looking for.

I am running the Inktomi mod from the resource centre to automaticaly send a user to fdse when no directory results are found.... it's super