8bpp scaling with Direct2d

I want to scale a 1920x1080 image to 1280x720. The source and destination buffers are both in main memory. Each pixel is 8 bits. The images are the Y component of a video frame.

For best performance, I want to avoid buffer copy. IWICBitmapScaler can do the scaling but is very slow. I'm trying Direct2D, but I can't get DrawBitmap to work for an A8 Render Target. I get hr = 0x88982f80 : The bitmap pixel format is unsupported. How can I use DrawBitmap with an A8 render target?