Here's a way how you would create DS/GBA fakes:
1) Acquire suitable digital footage or images of Doom3.
2) Transcode that footage into MP4 format for use with Play-Yan.
3) Have the Play-Yan display the footatge/image(s) on the DS.
4) Take pictures. The reflections of the photographer on the DS screens helps make the photo seem legit.
5) Do some photoshopping or use some other tricks to make the DS appear to output to both screens (since i think the play-yan won't output to both screens at once).
6) Crop the picture in such a way that you don't show the Play-Yan is attached to the DS at the bottom (WHAT'S THAT? THE PHOTO DOESN'T SHOW THE BOTTOM OF THE DS?! =O ).
The alternative: use a homebrew app to display the images, as KDR stated.
This also applies to the Twighlight Princess fakes that appeared on GameFaqs a while ago. Have some pre-rendered images created on your comp, then use built-in or external TV-output devices to display the image on a TV (or send the image from the comp to a firewire camcorder, which is displaying on a TV). Takes pictures of the TV, then brag on the internet. Alternatively, you can put the pre-rendered pics on a DVD or picture CD that could be display via compatible DVD players, or display via various digital cameras that come with TV-out.