The source has been reviewed, and no method is included with it.
I really doubt they'd include something to boot off SD card, as the diagnostics are internal and the ROM is not re-flashable.
To play video from SD, someone will have to port a video player to the open source image (iPod Linux's video player seems like a good canidate), and then that image will have will have to be burned to a (E)EPROM and then JB's ROM physically replaced with that, if you want to be completely independant from a PC.