|To roughly (very roughly, I don't actually understand this stuff) reprise:|
the custom video carts apparently have a boot kernel on them, and a video player; when one of the video carts is put in, it interfaces directly with the processor instead of the integrated OS.
There are people in the forum who, if I am reading things right, are past the point where they could create a proggy that would package video (not in WMV format of course) properly on the SD card for you, but they have no real interest in it, so probably won't do it.
In addition, I believe that the MP3 adapter cartridge for the juicebox is missing a lead for it to be able to work for video, anyway; so you would have to either A) modify a existing adapter or B) find someone willing to do it for you or C) add a 3rd party adapter.
I myself am hoping for some miraculous revelation to happen that will allow video to be played directly from a SD card in a standard MP3 adapter, but i'm not holding my breath (even though I did rush out and buy 10 juicebox's and MP3 kits because I figured it would just be a couple of days after the kernel was released before the video was figured out). in the meantime, I've taken one out of its shell and embedded it in the dash of my jeep with a 512mb SD card; makes a great off-roading shockproof soundsystem.