I don't have enough time to dig deep enough into the VideoPlayer. I just added the implementations of the methods EVRCustomPresenter::IsRateSupported, EVRCustomPresenter::GetSlowestRate and EVRCustomPresenter::GetFastestRate suggested in MSN, but this didn't help. The graph manager still doesn't accept another playback rate than 2x for seeking (and play = 1x, off course).
I think we would need 0.5, 1.0, 2.0, 4.0, 8.0, 16.0, 32.0, ..., and the negative rates.