Don't think it can be done in the timeframe given. Britain tried to pull the "There's no one here in this island, so we'll put our flag on it and claim it as ours" trick(which is what they did with the Falklands) with Trindade island in the late 19th century, but the Brazilian government took out the flag and politely reminded the British that Trindade was Brazilian territory. Doing this in an even bigger island, located in a place where Brazilian ships would certainly pass through, wasn't feasible even then, to say nothing of the 20th century. Last opportunity to pull this would be in the 1830's, I think.