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 5 for Windows and Rhino 5 for Mac. Plugin developer will have to use RhinoCommon (C# .NET); plugins authored in C/C++ are not supported on Rhino 5 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.