I am not sure, but I think for opening a dôjô you will Need the " brevet d'État d'éducateur sportif". Which is a teaching licence:
http://www.aikido.com.fr/Brevets-d-etat
In addition to this in France the grades that are relevant to teaching are given by the state (i.e. the ministry of education), as far as I know.
But I think it is related to the grades given by hombu in some way.
I recommend to talk to Christian Tissier. He is a very polite and humble person. I am sure he will try help you out.