Current Snapcraft requires a keyserver and a key ID to add a GPG key for an APT repository. This can be hard to use if the remote APT repository doesn’t publish their GPG key to a keyserver, and only has it hosted via a URL.
appimage-builder (see link below) seems to solve this approach just fine, and simply allows specifying a URL to fetch the GPG key from, without it needing to be hosted on a keyserver (though it indirectly ends up supporting that way as well anyway).
I’m mainly needing this so I can add some APT repositories during the building of my Snap. The only other way I’m thinking of right now is building via an AppImage and then copying that into the Snap, but I’d like to avoid that if I could.