I went through this a few months ago. It is totally disgusting! It is a decision by some Florida Bureaucrat and not an act of law. They treated me VERY disrespectfully, attempted to take away any value of my purchase, and only wanted to exercise their perceived power.
Perhaps this will help you. Long thread, but it took me almost 4 months to work through it and a LOT of unnecessary expenses.
Wife wants me to do the unthinkable due to the Florida DMV debacle!
I can definitely tell you that head bureaucrat indeed wanted me to rat out friends that owned other humvees so that he could pull back their plates. The list of obscenities I invented for this POS was historic.
And, no, you can only register a humvee under a Historic Military tag after passing a in person inspection and if ANY change was made to the vehicle, even if it is for safety like replacing bald tires, the bureaucrat will mark the title as "off-road" only and you will not be able title it in Florida. They will not register it as an antique "collectable" either.
Whatever you do, do NOT take your out-of-state title to a Florida DMV as they are likely to invalidate your out-of-state title for you, like they did for me.