Answering my own question here, since I found it via digging in and modifying one of Steve’s sample - this will allow to access a lot of custom settings:
import Rhino
settings = Rhino.PlugIns.PlugIn.GetPluginSettings(Rhino.RhinoApp.CurrentRhinoId, False)
s = settings.GetChild("Options").GetChild("General")
print s.GetBool("MiddleMouseViewManipulationMode")