Encountered the same problem, any answer?
ArcGIS Classes (Featureclass, Layers, Fields, Maps, etc.,.) in arcpy do not play well with being transferred between python threads due to python's pickling. I haven't met anyone yet that has created their own pickling class to transfer the esri classes either. I think if you can figure out how to pickle the arcpy data class being transferred, you'll be able to get closer to a solution.