Thanks both, we'll have a Green JR Pass and we are well familiar with the trains. We'll come from Osaka so Sakura + Ltd Express Sonic will be the way. We'll probably use Beppu as a base for 4 nights and do 2 days nothing and 2 days some exploring around the area (using the car) and then off to Fukuoka as a base for a few days before taking SQ direct back to SIN. We'll save southern Kyushu for another time.

Seven Stars is nice, but a pain to book and my wife would be grumpy being locked up on a train. I'd be nicknamed "train otaku" in no time.