There is a project named SampleExternalModule in the mojoportal-complete.sln, however its not very complex and therefore not as good an example as just studying real features to learn how to implement your own features. The mojoPortal.Features projects and the WebStore projects are good examples of complex features implemented as external projects. I recommend you structure your own custom projects the same way these are structured and study the code in those to see how to implement your own.
Hope it helps,
Joe