So there is this tool:
https://mariomasta64.me/dsi-shiz/applications/windows/DS_Demo_DeCompress.zip
Use DSBuff to unpack the NDS rom:
https://www.romhacking.net/utilities/1214/
If you can find the roms of the DS Download Station cartridges (
), use the above tool to convert the demo files (I think in the mkdir) from the game. Then you can play them on a Flash Card.
If you are talking playing Demos from retail games, you just need to locate the .srl file (basically an alt name for .nds) and rename the type from: .srl to .nds
For example, Elte Beat Agents demos are located: data\data\rom\
- trial.srl = Play the first two sections of the Walkie Talkie Song
- tutorial.srl = Play the Tutorial level
Sonic Rush has them located at: data\mb\
When I load them in the emulator, a communication error appears, so they won't work.
Advance Wars - Dual Strike: data\boot\
- mb_combat_sgn.srl = Combat Demo
Ridge Racer DS: data\mb\
Since the user has to select which course to download, "main.srl" won't boot.
Hope this helps.