I'm pretty sure this isn't possible.
On the iPhone you launch other applications by using a custom URL scheme. A good list of these can be found here: http://wiki.akosma.com/IPhone_URL_Schemes
So for example to launch Safari:
NSString *stringURL = @"http://my.url.com/";
NSURL *url = [NSURL URLWithString:stringURL];
[[UIApplication sharedApplication] openURL:url];
The iPhone then decides to launch Safari as the URL scheme used is http. As you said in your question you can just use the correct URL to launch a search engine, so for Google:
http://www.google.com/search?q=MY_SEARCH_TERM
What would be ideal would be if Apple provided a URL scheme, such as search:// which launched the query in a search engine. However, this sadly isn't implemented.
I think the best way to "solve" your problem would be to allow the user to choose which search engine they'd like to use in your app. Then, when launching Safari you can use the appropriate URL. The added bonus is that you can also include search engines which Apple doesn't.