<edit>Also, UIWebView doesn't exist on Mac OS X, only on iOS. On OSX there's just WebView, which is more powerful, but also doesn't support XAML. </edit>
WPF Browser Applications are backed up by a .NET EXE... That's a big fat no. It's not a browser feature, it's something driven by a .NET browser plugin (similar to click-once) that is Windows-only.
You could read in the XAML, parse it and then programmatically add the elements to the view. I'm looking at doing something like this for a project. We're not going to use XAML but something like it.