Good question - the whole time while I was reading the book, I couldn't help thinking about America, since they'll probably join in before the war is over. (And I guess if Westerfeld does have them enter the war, it's a safe bet that they'll be Darwinist.)
Wikipedia says
When World War 1 broke out in 1914, all Dominions of the British Empire, including Canada, were called upon by Great Britain to fight on her behalf
So, considering the fact that at the time Canada was still basically under Britain's control, they more than likely would have Darwinists.
And during WWI, Japan apparently sided with the Entente, so perhaps they would have been officaly Darwinist?
Korea was neutral (which makes me wonder - is Switzerland completely neutral, or does it lean more towards Clankers/Darwinists?).
China, I'm not so sure about....they declared neutrality until 1917, when they (like the Americans) joined and fought against the Central Powers/Clankers. But weren't there many different spheres of influence from different countries at the time? I'm sure that would affect their stance on the Clanker/Darwinist fight.
But, my assumptions are limited because they're only based on politics and "who supported who."