If you have your US system BIOS modded it will automatically boot-up in japanese. Or so I'm told, even if it's piggy-backed (debug bios chip soldered on-top of original bios chip)
I suppose a way around this is to have a switch installed for which region you want it to boot up in. I wonder if you can have a switch installed that activates either US boot-up OR debug-bios boot-up? I'd go for that if ever I was going to have my US AES modded.