If I understand your question, you would like to make the model's head and the back of the hair invisible to your main camera.  Firstly, if the model's head is going to be invisible, then there is no need to load the model at all.  You can load the hair only for the specific model it is intended.  Secondly, the way to render the hair without the back is uncheck cast shadows in the Hair Properties and in your render settings check 'remove backfacing polygons'.  That should do the trick.