Britain garrisoned & effectively administered Madeira for part of the Napoleonic Wars. Maybe the British government (prompted by the Admiralty, or by lovers of Madeira wine, or whatever...) decides that they would rather like to keep Madeira afterwards, and "persuades" Portugal to accept continued British control there (possibly still with nominal Portuguese ownership?) in exchange for compensation elsewhere which could include incorporating the former French Guiana into Brazil?I'm guessing it's borderline ASB to actually have their ally Britain throw them a bone (Britain kept everything it wanted).