Summary: .NET plug-ins that run on both Windows and Mac are possible and in development.
It is possible to author plugins that will run on both Rhino for Windows and Rhino for Mac. Plugin developer will have to use RhinoCommon (C# .NET); plugins authored in C/C++ are not supported on Rhino for Mac.
For a list of plugins supported on Rhino for Mac, please see Food4Rhino.
Developer info can be found in the RhinoCommon section of our developer guides.