Are you referring to the property page available on the sample dshow filters? If so, those filters are only samples and
do not expose all the features of the Media SDK. The encoder does have the ability to set a particular frame rates and the setting is found in the mfxFrameInfo structure.
Set the encoders frame rate by setting mfxFrameInfo->FrameRateExtN / mfxFrameInfo->FrameRateExtD.
So, to set 30fps: mfxFrameInfo->FrameRateExtN = 30 and mfxFrameInfo->FrameRateExtD = 1
Perhaps you can enhance the samples to add this functionality.