It would be nice to be able to specify a custom user agent string. Debug > User Agent > (Manage Settings)
Thanks for filing the enhancement request. The Debug menu is a feature of the Safari application, not the WebKit framework (<http://webkit.org/blog/101/back-to-basics/>), so your request should be addressed to Apple via the Apple Bug Reporter at <http://bugreport.apple.com/> (requires ADC membership; you can sign up for free online membership).