The Malagasy Kingdom was still a potent local force up until the late 19th century. Madagascar wouldn't have been France's to trade away, and would require the British to play France's role in colonialism there.
But making whites the majority anywhere in sub-Saharan Africa is beyond challenging. Madagascar is better than most places climate-wise, but the Malagasy were also one of the most organised peoples in the region, totally unlike the tribal societies and chiefdoms in Kenya and Rhodesia. Britain would need to be prepared to pay a huge cost if they want to conquer the island, assuming the Malagasy mount an effective resistance. The French might as well arm Madagascar to protect their interests there and increase their influence, unless this is post-Berlin Conference. And if they do it during Radama II's short rule, they've probably just given Madagascar a chance to pull a Thailand and effectively modernise, since Radama was on the tier of some of the late 19th century Thai kings in his vision.