Just a quick update; after some re-reading of the MPEG2 spec, debugging, and clean up I've finally got correct output for the MC stage for progressive video clips that use frame based and field based motion prediction. There are two other motion prediction methods, 16x8 and dual-prime, but they don't seem to be too common and shouldn't be too hard to implement anyway. It took a bit of tweaking, but comparing the output to that of other media players I see no difference, which means one hurdle down. Next steps are to revisit IDCT and start working with real hardware.
Here are some screen grabs from various test clips: