See this other thread
http://chdk.setepontos.com/index.php/topic,1596.msg14444.html#msg14444 for a video mode MD script and the following discussion about a problem with md_detect_motion and the video modes.
It seems that while the camera is in video mode, we're unable to trigger from the bottom half of the LCD and that masks behave erratically as well. The test mode of the said script is a good way to test this, but I believe any motion detect script will fail. The video submode seems to affect how things go as well.
Any ideas why this could be happening? Is the LCD buffer contents somehow different in video mode?
(For clarity: I'm not really talking about MX3 builds, but recent builds such as Allbest 50 which have MX3's motion detector built in).