I've successfully set the statuscode in Dynamics365 - obvs it has to be one of the allowable integer values of the option set (not the textual value). Have you/can you use something like XRM Toolbox to query the Dynamics instance you can then see the list of actual allowable values.
The only thing of note, is that I dont think Dynamics likes it when you try to insert a NEW record that is inactive (though this would be the statecode), you have to insert as active and then immediately set it as inactive.
