I can't guarantee this will work, but you could search in your save file for all mentions of
["Human",25],["halfbreeds",5],["Elf",3]which are the default spawn rates for races in plains; then change it to whatever you want. You could replace 'Human' with 'Orc' to get mostly orcs in Aliron, instead of mostly humans. You will probably have to replace more than one instance.
Backup your save first.