I'm curious that they recommend remapping the hostname so that you actually just can't access yahoo (incidentally, that should break SSL for their website if you access it via the yahoo address) instead of, say, editing ~/Library/Safari/Configurations.plist.signed. (Not that I've tried editing that file; it's just that it very clearly defines the search engines that are already in use, and you could perhaps add another or else change the URL that it uses for one of the built-ins).

I tried editing Configurations.plist.signed and it did nothing. I don't think it gets loaded if the signature doesn't validate. By the way, all of the urls in Configurations.plist.signed are http: not https: so ssl certificates don't come in to play.

I choose this method rather than a toolbar extension because I didn't want to lose 20 pixels of vertical content space. To each his own.