Forcing RGB output from VMR9 Renderless Mode

Question

I've setup customer Allocator-Presenter class based on the vmr9allocator sample in the SDK. Everything seems to be working for the most part, but I'm having a problem trying to get RGB output from it. I have one file in particular I've been testing that always results in my IVMRSurfaceAllocatorNotify9::InitializeDevice method being called with a surface format of D3DFMT_UYVY. I've tried using the IVMRMixerControl9::SetMixingPrefs interface off of the VMR9 filter to set MixerPref_RenderTargetRGB but when I do this SetMixingPrefs always returns E_INVALIDARG. Is this the right way to go about this (i.e. is there a way to force a particular format like this)? Or do I need to do something upstream from the VMR9 filter to set the desired color-space, something like what is suggested here: http://social.msdn.microsoft.com/Forums/en-US/windowsdirectshowdevelopment/thread/3fc53111-3152-4c12-9fa5-e4bbb666ae9a .