Run metro app from custom shell

I'd like to create a custom shell (WinForms) for Windows 8 Embedded Standard (Release Preview) which allows the user to run, install, or update applications (can be desktop and/or metro apps).

Therefore I replaced explorer.exe in the registry with the path to my custom shell, which works pretty well. When I attempt to run metro style apps using the IApplicationActivationManager (see implementation detail here), I get this error message:

The app manager is required to activate applications, but is not running. (Exception from HRESULT: 0x80270257)

The event log doesn't contain any additional information (neither in Application nor in TWinUI).

Metro style apps run well when explorer.exe is used as a shell. I didn't find a process or service yet which I could simply start to allow execution of metro apps (I tried to run WWAHost.exe, but this fails with the error message The system cannot execute the specified program.). Starting explorer.exe manually doesn't change anything.

I'm afraid I don't have a solution for you, but seeing as this exception is unheard of (Google it, I dare you) I figured I would point out that I run into the exact same issue in the Visual Studio 2012 Designer trying to run it as a RemoteApp (installed on a Win8 VM in a VDI environment).

!The full exception as thrown by VS2012 reads1