That doesn't work, you need to set it afterwards. Is there an onchunkloaded hook you could use? That would be easiest. And it would avoid extra strain - only change the biomes when somebody is there to see the change.
HookChunkAvailable looks perfect:
http://mc-server.xoft.cz/LuaAPI/OnChunkAvailable.html
HookChunkAvailable looks perfect:
http://mc-server.xoft.cz/LuaAPI/OnChunkAvailable.html