There is no loophole, except that the CFR says that if you MAINTAIN a home in a state, you are considered to be a resident of that state during the time that you are staying in the home that you maintain. Since we here in Florida have a large snowbird population, that covers them quite well. (27CFR478.11)
The question would be what does it mean to maintain a home? The CFR gives some guidance on that. It specifically says that a person who owns property (like a hunting cabin) is not maintaining a home. Likewise, renting a timeshare would not be maintaining a home. The CFR does say that a person who has a home that he only stays in on the weekends and the summer would be a resident during the time he is in that vacation home, therefore a person in the situation described by the OP would not be breaking the law by purchasing a handgun or a long gun while he is here, whether that is face to face, or through a dealer, as the regulation clearly considers him to be a resident.
The problem comes when you go to prove your residency to the dealer. The regulation states that the proof of residency must be a government issued identification that has your name, DOB, and address on it, or any combination of government issued documents that state those items of information. In the OP's case, a DL from his other state and a tax notice from the tax assessor's office may be sufficient, but a state ID or CCw would be easiest and present the least hassle. Since you are not required to present such ID in a FTF transaction, you could do that legally without even having an ID, as long as you maintain a home in the state.
However, a state ID or CCw with a Florida address on it does not make you a resident, as you can obtain those without proving that you are a resident. Residency is the key.