Sorry for the silence - we will investigate issue, but you are very welcome to investigate along with us.
You will find the code for MvcPlayer in ~/App_Code/Composite/AspNet/MvcPlayer/Player.cs - it is just 159 lines. "public static XDocument Render(string Path)" is the entry point from Composite C1 to this function.
If the problem seems to stem from the Composite C1 core, try running with the source code version, set a break point in the MvcPlayer and backtrack into the code.
We will keep you updated on what we find :)