I have finished the GBA Movie Player firmware hack. It will allow you to boot a single NDS or GBA file from your CF card. More details are in the zip file. It will only load homebrew NDS files, commercial demos do not work!
This is intended to be used with a file select program, but none exists. Oh, and Dwedit has said that it corrupted his GBAMP, but I haven't had this problem. Use at your own risk.