Since I haven't written much "UnityScript" code myself, I'd appreciate your comments about whether you think it's possible (or worth the effort) for a tool like Browserify/Unityify to translate JavaScript code so it runs in Unity3D, or for programmers to write portable modules in a dialect of JavaScript that will run in both.

Maybe Unity3D themselves could help, by fixing problems with UnityScript that make it hard to run standard JavaScript, and providing hooks in their compiler pipeline that enable passing JavaScript libraries through with minimal modification when you're targeting the web browser and WebGL.