Mplayer , the videoplayer of Wiimc has some good deinterlace filters (and many others) wich can be added via the command line or .conf file but Tantric has no intention of adding .conf file support and even if you could add the deinterlace filter (yadif, mcdeint, tfields etc), the framerate will drop so much that the movie will turn into an slideshow of images…
My favourite filter is yadif=1 wich outputs 1 frame for each field resulting 50 fps from a 25 fps interlaced movie. At 50 fps the video playback is very smooth (you can try the filter using windows version of mplayer because wii processor does not have the power to apply the filter in realtime, or it has the power but the code isn’t optimized)
But since the wii video-output is interlaced there must be a way to play interlaced movies without first deinterlacing and then interlacing them again…