It's not really as outlandish as it seems to have Canada as a permanent security council member in the immediate aftermath of the Second World War. After all, when the war was over Canada was one of the most powerful military forces in the world, with a larger navy than the Soviet Union, France and the ROC, and a larger air force than France and the ROC, and an enviable army too.
How about this? Round about the time that the Soviet Union is pressing for the Ukrainian and Belarusian SSRs to be given veto powers, and the United States countering with its own demands for State veto powers, to have the United Kingdom suggest that it too is entitled to extra veto powers, and these will be in the form of Commonwealth members, such as Canada.
Too implausible?