In North Carolina where I live, the size of the gun, or the mode of carry isn't really define when it comes to what constitutes "concealed". Concealed is defined here as "in plain view, or readily recognizable as a firearm".
However, in the "NC Firearms Laws" booklet that the State Attorney General puts out, they DO use the specific example of a "beltbuckle derringer", which would be similar to the NAA revolvers you are talking about. The NC AG clearly states that such a firearm WOULD be considered "concealed" by the courts, because it's not "readily recognizable as a firearm". To an untrained eye, it could be just an ornamental buckle, or even just a fancy carving on the buckle.
But this varies from state to state, depending on your specific laws and the rulings and opinions of your courts and Attorney General.
If you let us know where you are located (put it in your profile), we can give you a more concrete answer...