Legal Definitions May Allow you to ride unregistered.
The 2 key words are "Transportation" and "Carrier".
The 2014 Florida Statutes
Title XXIII
MOTOR VEHICLES
Chapter 320
MOTOR VEHICLE LICENSES
View Entire Chapter
320.01 Definitions, general.As used in the Florida Statutes, except as otherwise provided, the term:
(1) "Motor vehicle" means:
(a) An automobile, motorcycle, truck, trailer, semitrailer, truck tractor and semitrailer combination, or any other vehicle operated on the roads of this state, used to transport persons or property, and propelled by power other than muscular power, but the term does not include traction engines, road rollers, special mobile equipment as defined in s. 316.003(48), vehicles that run only upon a track, bicycles, swamp buggies, or mopeds.
If it does not transport, it is not a "Motor vehicle". Not my opinion, the legal definition.
(32) "Motor carrier" means any person owning, controlling, operating, or managing any motor vehicle used to transport persons or property over any public highway.
What does "Carrier" mean?
carrier
n. in general, any person or business which transports property or people by any means of conveyance (truck, auto, taxi, bus, airplane, railroad, ship), almost always for a charge. The carrier is the transportation system and not the owner or operator of the system. There are two types of carriers: common carrier (in the regular business or a public utility of transportation) and a private carrier (a party not in the business, but agrees to make a delivery or carry a passenger in a specific instance). Regular transportation systems are regulated by states and by the Interstate Commerce Commission if they cross state lines. (See: common carrier, private carrier)
Are you a "Carrier" on your MB who transports property or people for a charge?
If not, your MB is not a "Motor Vehicle".
Again, not my opinion, just the words as stated.