I don't know how to do it in UIWebView, but you can definitely set user-agent using NSURLRequest directly and parsing the data yourself. Something like:
Code:
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:whateverURL];
NSMutableDictionary *headers = [NSMutableDictionary dictionary];
[headers setObject:@"iPad (like giant iPod touch); Mozilla; Chrome; Whatever" forKey:@"User-Agent"];
[request setAllHTTPHeaderFields:headers];
NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:NULL error:NULL];
(disregard all the mutables, they're from some other code).
If you must show it in a webview, you might do something like logging in using the URLRequest, then saving the cookies and using them later for the webview.