Romhacks are usually distributed as IPS files and not full ROMs. These are totally legal, and it's why sites like https://metroidconstruction.com/hacks.php can exist.

There's no good Linux CLI utility for this, but... It's okay, because someone made it into a static webpage. You can download this .html file and use it offline.

I'll be enjoying MDBTroid, an improvement on the original NES game. <3