What you're describing is exactly how the QT Framework works, it's cross platform, and the Ruby bindings already exist. It also enjoys the advantage of being open-source, although the QT Designer IDE is proprietary. So just as you can write and compile C# code for the .NET framework without Visual Studio, the task is greatly simplified with the native IDE.
GUI work is well outside my day to day development however, so I'm not sure if it could be made to work with the Sketchup API. I'll defer such decisions to those better qualified to make them. π