A few things to consider. Plugins are installed in vnl/lib/{python2.7}/site-packages/AddOns and most of them are provided in source code, yes.
Then it depends mostly on which kind of plugin you are after - Builder import/export, or LabFloor filters. But you are right, we haven't published the API (the Viewer and LabFloor ones are still being developed and may change), or good examples. It will come, I promise!
Have a look at the CASTEPFilters, they are very barebones and easy to understand, by providing the minimal required functionality.