I think you are all insane. I don't understand why you would go above 240 grit. A bat will get seam marks, red marks, scrape marks all over it and you want to achieve a finish a French polisher would be proud of. I look after my gear well but this is unnecessary.
Whats the best way to sand concaved backs, that's what I've always struggled with, bearing in mind I do it in a manshed with limited tools.