Here are instructions for creating custom nations and the custom nations warinfo file editor.
These are provided as is for those who want to create their own custom nations. The documentation covers the essentials but may be a little sparse in places, but I will rather post it now than wait for the documentation to get perfect. Users are encouraged to add to the documentation file and post updated versions, it is a rtf file for simplicity.
Just download and unzip in the RTW directory. Read the info and run BCWarEditor.exe to edit warinfo data for the custom nation.
Thanks to GalagaGalaxian for helping test the editor.