Microsoft has enabled several, previously-unavailable thread-creation and management APIs for recent WinRT apis, specifically for Windows 8.1 and Windows Phone 8.1.  These APIs had previously been made private (for WinRT apps).

Such API calls include, potentially among others:
- CreateThread (docs at http://msdn.microsoft.com/en-US/library/windows/apps/ms682453.aspx )
- SetThreadPriority (this appears to not be functional on Windows Phone 8.1, however, according to MSDN docs at http://msdn.microsoft.com/en-US/library/windows/apps/ms686277.aspx )

Such a move should reduce the amount of custom code needed to get the WinRT port of SDL running, and in some cases, may enable thread-priority changing, which was previously unavailable in SDL/WinRT.

Be sure to check and see if these API additions require particular versions of MSVC.  For example, the Win32 Sleep function was also made available for Windows 8.1 and Windows Phone 8.1, but only as of Visual C++ Update 4.