Greg Pierce put together a hacky way to use x-callback-url in Safari, despite Safari's lack of support.
Not sure whether to call this a tip or a hack. Either way, this post demonstrates a way to mimic Google Chrome on iOS’s x-callback-url support in Mobile Safari through the use of some javascript and an intermediate static html page.
Always security conscious, Greg notes that you are welcome to duplicate the HTML file and host it on your own server. That way, no one besides the NSA can see what you're doing.