True, Madagascar's population base of roughly 2M in 1850 isn't much to work with; but it's still markedly more than Zanzibar. And Madagascar's actually one of the few places in Africa which had a decent shot when it came to industrial resources, such as coal deposits:
Not much difference between them and Japan and/or Great Britain in that regard, is there? And Madagascar also now has proven recoverable oil deposits of over 19.1 billion barrels- albeit with more than half of these locked in oil sands, and roughly 2.4 billion barrels of oil which could be utilized in the industrialization period; still, far better than the initial 400 million barrels of oil reserves which Japan had to make do with. And Madagascar is one of the most mineral abundant nations on earth, with numerous and substantial deposits of both industrial and metallic minerals, including, but not limited to: ilmenite, graphite, pyrolusite, limestone, gypsum, dolomite, silica, mica, titanium, quartz, gold, platinum group, silver, iron, copper, zinc, nickel, cobalt, chromite, coal, oil and uranium, along with a great number of precious and semi-precious stones (with the world's richest sapphire deposits). If they'd had the political organisation and the will to do so, it's one of the few African nations which had the capacity to industrialise and 'pull a Meiji'. As for the backwardness of the country- well, given more than 2 yrs to implement his progressive policies more completely over a longer period of time, Radama II could have done something about that. And the complete disunity of the island before the 19th century wasn't so complete- the Merina Kingdom dated back to the mid 16th century, and it was widely acknowledged that the Merina monarch was the king of Madagascar from the dawn of the 18th century onward. It may not be very likely, and either an earlier POD or a significant amount of good fortune may be needed to make it plausible. But unlike several other candidates, it was possible.