We have released a "preview" version of Studio for WinRT XAML that supports Visual Studio 2013 preview and Windows 8.1 preview. You can download it below:
http://www.componentone.com/Downloads/Download/?downloadID=98
It installs side-by-side to our 8.0 controls. There is a known issue regarding design time support. When you drop 8.1 controls to your page, the control may appear blank on the design surface. This does not affect run-time behavior.