A few years ago (2011-2012) I went through the exercise of having detailed discussions with both Louisiana Fish & Wildlife and the local Marine Safety Office on this subject. The instructions I got were as follows: Due to a change in state law, I now needed to register the boat with the state; but, due to a conflict between the state requirement to display the registration and the USCG prohibition against same, I could not meet the state display reqs - the bureaucratic answer was to state register the boat, then hide the state registration decal/# from exterior view. So, my state registration decal resides in the binder that contains the USCG cert, insurance papers, etc.
I suggested, in a conference call with both parties, that LDNR and COTP New Orleans and/or the District dog issue a letter/instruction that would memorialize that understanding and assist boater compliance. They suggested I could hang up now.