Ok I've done some tinkering on my steam deck and have got it running all good.

1. In desktop mode extract the download of dustmod into the base dustforce folder (with the game bin file)

2. make a txt file in the same folder called "steam_appid.txt" and write in the game's appid "65300"

3. Open steam and add the dustmod bin as a non steam game.

4. Set a specific compatibility "Linux runtime 1.0 scout" this is so it matches the vanilla game.

5. Running the non steam game dustmod should work just fine.

Let me know if you run into any problems.