Hello,
Could you add the line MimeType=application/vnd.nintendo.snes.rom; in src/linux/zsnes.desktop?
So ZSNES would be listed when I ask my package manager which application can open a SNES ROM. It could even be installed automatically when I try to open it.
See:
http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html
https://www.iana.org/assignments/media-types/application/vnd.nintendo.snes.rom
Thank you.