I haven't tried it personally, but from what I can tell, if you have a binary you can give a path to (and setting up env vars if needed in the "launch arguments"), you should be able to add it as a "non-Steam" game, and anything set up to start automatically via systemd seems to be properly started up in game mode, so I don't see why this wouldn't work!

Haven't tried adding Nix packages as non-Steam games, but you can use Home Manager to create user systemd rules.