The decomp port is nice too.

It’s not something I’d seen before, but I found software that already does donation DLC readily available on Steam. Here is an example.

Nintendo of America often used pixel art for their own box art early on in the NES era. It was similar to the in game graphics, but usually more detailed. See Metroid’s original artwork. If there was a requirement for third parties, perhaps it could be met by simply including screenshots on the back.