Since the OSD pops up when hitting a keyboard arrow or space bar even if LAV or madVR aren't used, I'm under the impression MC handles the OSD, not the filters.
I'm not asking for 3D support, I'm merely asking them to make a SBS/OU alternative OSD, which for SBS would be compressed 50% and duplicated in the width, and for OU compressed 50% and duplicated in the height.
Next step would be the same for the subtitle renderer. EDIT: Not very important, since when you rip a 3D Blu-ray to SBS/OU, the subtitles are also converted, so playing a ripped 3D Blu-ray with subtitles works in MC. Making a 3D mode for the subtitle renderer would only apply to using external subtitles, for example if you download subtitles in a different language than is available on the disc. Maybe it's needed to display 3D TV subtitles correctly when using MC with a DVB receiver card, I wouldn't know.
SBS/OU files require no 3D support whatsoever to play, they're normal 1080p (or 720p) files that simply shows a double picture until you tell the 3D TV/projector to convert into 3D, or it detects it automatically. It's just that when the 3D TV is in 3D mode, everything on the screen, such as the OSD, which are not doubled like the picture, will be all over the place.