That's a good question. I have no idea. If it uses the EXACT same system partition then no problems, but if it's different than we'll need a different pre-rooted image. Maybe rbox knows if the non US models are different. There really isn't an easy way to check that I know of besides MD5SUM the partition which requires at least restricted root access.
Thanks again! So I hope that rbox will know an answer to that. I'll receive my Fire TV today and I'd be willing to experiment a bit but wouldn't want to take the risk of bricking my device.
Greets, Christian