If you don't need the APIs that interact with the local system, you can already build for Electron and for the browser easily. If you need those APIs, however, there's no other possibility because of the browser sandbox.
It's possible to have a native (node?) app for the local side, which talks to a browser extension. This way you don't have to run multiple instances of the browser runtime. There's someone writing a full-featured, Patchwork-like SSB client in Firefox this way, but unfortunately I can't remember the name.
