Video overlay / VMRMixerBitmap doesn't show

Ok, I'm going to include a code snippet (with all error handling and cleanup removed) to help with this one. I'm using the VMR to render the video stream, and I want to overlay a bitmap image. As I understand, the Mixer bitmap serves this purpose. The video displays, but the overlay doesn't, and I cant see why - I have scratched my head for a while over this one, and searched the internet too - it seems to me that it should work (I'm using managed extensions to C++ btw). I'd be grateful if someone could point out the error of my ways...

This is how I obtain the mixer bitmap:

 pin_ptr<IVMRMixerBitmap9*> pPinnedVMRMixerBitmap9 = &pVMRMixerBitmap9;
 hResult = pVideoMixingRenderer->QueryInterface(IID_IVMRMixerBitmap, (void**)(&pPinnedVMRMixerBitmap9));

 pVMRMixerBitmap9 = (IVMRMixerBitmap9 *)pPinnedVMRMixerBitmap9;
 InitialiseVMRMixerBitmap();

And this is the  InitialiseVMRMixerBitmap() body:

void InitialiseVMRMixerBitmap()
{
 HRESULT hResult = NOERROR;
 Graphics ^pGraphics = nullptr;
 IntPtr pBitmapHDeviceContext = IntPtr::Zero;
 IntPtr pHBitmap = IntPtr::Zero;
 HDC hDeviceContext = (HDC)0;
      
 pVMR9AlphaBitmap = new VMR9AlphaBitmap();
      
 /// Get current Alpha Bitmap parameters
 hResult = pVMRMixerBitmap9->GetAlphaBitmapParameters(pVMR9AlphaBitmap);

 /// Put this into a know condition
 memset(pVMR9AlphaBitmap, 0, sizeof(VMR9AlphaBitmap));

 /// Disable the them by setting this enumeration and writing them back to the mixer
 pVMR9AlphaBitmap->dwFlags = VMR9AlphaBitmap_Disable;
 hResult = pVMRMixerBitmap9->UpdateAlphaBitmapParameters(pVMR9AlphaBitmap);

 /// This is the image to be overlayed
 Bitmap ^pBitmap = gcnew Bitmap(320, 240, PixelFormat::Format24bppRgb);
 Graphics ^pTestGraphics = Graphics::FromImage(pBitmap);
 pTestGraphics->DrawLine(gcnew Pen(Color::Red), 0, 0, pBitmap->Width, pBitmap->Height);

 Color keyColor = Color::White;
            
 /// Here we have some managed GDI - native voodoo
 pGraphics = Graphics::FromImage(pBitmap);
 pBitmapHDeviceContext = pGraphics->GetHdc();
 hDeviceContext = CreateCompatibleDC((HDC)pBitmapHDeviceContext.ToPointer());
 pHBitmap = pBitmap->GetHbitmap();
 SelectObject(hDeviceContext, (HGDIOBJ)pHBitmap.ToPointer());

 /// Now we can configure the alpha bitmap to use the GDI device context
 pVMR9AlphaBitmap = new VMR9AlphaBitmap();
            
 /// Bitwise combination of flags from the VMR9AlphaBitmapFlags enumeration type.
 pVMR9AlphaBitmap->dwFlags = VMR9AlphaBitmap_hDC | VMR9AlphaBitmap_SrcColorKey | VMR9AlphaBitmap_FilterMode;
            
 /// Specifies the handle to the device context for the bitmap. Specify NULL if the bitmap is located in a Direct3D surface.
 pVMR9AlphaBitmap->hdc = hDeviceContext;

 /// Specifies the source rectangle in either the GDI device context or the DirectDraw surface.
 pVMR9AlphaBitmap->rSrc.top = 0;
 pVMR9AlphaBitmap->rSrc.left = 0;
 pVMR9AlphaBitmap->rSrc.bottom = (LONG)pBitmap->Height;
 pVMR9AlphaBitmap->rSrc.right = (LONG)pBitmap->Width;

 /// Specifies the destination rectangle in composition space.
 pVMR9AlphaBitmap->rDest.top = 0;
 pVMR9AlphaBitmap->rDest.left = 0;
 pVMR9AlphaBitmap->rDest.bottom = 1;
 pVMR9AlphaBitmap->rDest.right = 1;
            
 /// Specifies the source color key.      
 pVMR9AlphaBitmap->clrSrcKey = ColorTranslator::ToWin32(keyColor);
 pVMR9AlphaBitmap->pDDS = NULL;
            
 /// Specifies flags from the VMRMixerPrefs enumeration, which control how the VMR mixes the image.
 /// The MixerPref_PointFiltering flag is particularly useful for images that contain text and do
 /// not need to be stretched prior to mixing.
 pVMR9AlphaBitmap->dwFilterMode = MixerPref_PointFiltering;
            
 /// Specifies the alpha blending value; must be a value from 0.0 to 1.0 (inclusive).
 pVMR9AlphaBitmap->fAlpha = 1.0;

 /// To remove the bitmap, set the VMR9AlphaBitmap_Disable flag in the VMR9AlphaBitmap structure
 /// and call SetAlphaBitmap again.
 hResult = pVMRMixerBitmap9->SetAlphaBitmap(pVMR9AlphaBitmap);
            
 pVMRMixerBitmap9->UpdateAlphaBitmapParameters(pVMR9AlphaBitmap);
}