I think the request is for something a bit like this as a plugin.
I think the request is for there to be a way to specify a custom "application" to handle files of a certain type (extension? MIME type?), such that Orion triggers that app to open to handle the file/URI (which?) rather than handle it internally.
I am not sure how it should work - not something I've used before. I am guessing the suggestion is Orion would spawn the "chosen" default app for the file, and then pass the parameter of the URL that was right-clicked to the app as a CLI argument? (I'm not sure how that works on Mac; that's how I envisage it on Linux/Unix)