How can I create a button with a UIActivityIndicator in my navigation bar with the same style as normal buttons?

All of the examples I've seen on here and other sites involved creating a UIActivityIndicatorView and loading it with something like:

self.navigationItem.rightBarButtonItem = [[[UIBarButtonItem alloc] 
                   initWithCustomView:myActivityIndicatorView 
                   target:nil 
                   action:nil] 
                  autorelease];

However, that just creates a plain activity indicator in the navigation bar. What I want to do is have a button that looks just like the normal UIBarButtonSystemItem buttons but with an activity indicator instead of one of the default images. I've tried doing initWithImage and initWithTitle with nil images or titles and then adding the activity indicator as a subview, but that doesn't work.

Any ideas?