Apache Cordova is a project which has support for Mac, Linux, Windows, iOS, Android, and more. (In some cases, platforms are still in beta.) Cordova is basically an embedded browser that lets you write webapps that install locally.
One interesting feature is that you can whitelist/blacklist network requests.
Another thing to consider: Mozilla’s Servo project is all in Rust. That’s their new web renderer. It might be nice to use that, but in going that route you’d lose out on all the platform support Cordova already has. (In addition, Servo only handles layout, not networking or JS.)
Just wanted to mention this. Looking forward to seeing what comes of the Launcher.