With apologies to @tennantje on GitHub, I have released a plugin that should resolve compatibility with Deluge.app on macOS (OS X).
It does this by modifying the system
$PATH when the plugin is initialized to reference the deluge package that is contained within /Applications/Deluge.app. The contrived name of the plugin is to ensure it runs before the normal Deluge plugin, so that when that plugin tries to find the deluge package, it succeeds.
Install it by downloading from GitHub and placing the file in
~/.flexget/plugins. I also added a wiki page for it.