You already can perform searches in the URL address bar, only you need to preface them with the shortcut/keyword for the search. E.g. typing
google OmniWeb will do a google search for OmniWeb for me, similarly
mu OmniWeb for a search at
www.macupdate.com. This is obviously far more powerful than what you suggest yourself as you also have control over what type of search is performed rather than it always being for one site such as Google. You can also edit the shortcuts/keywords to your own taste, so e.g. replace "
google OmniWeb" with "
g OmniWeb". With the exception of not being able to easily change the search engine in the search field using the keyboard, OmniWeb's search feature is extremely powerful and easy to use and is one of its key features that makes it stand out against the rest (Firefox is OK, but adding additional search engines and terms is a pain compared to OmniWeb's click a field, click a button, define your shortcut, done approach).