In around 2000-2001 there was a mulch-disciplined billiard event where some of the world's best in each sport were invited to Sydney, Australia to play off.
The format was US 8 ball, US 9-Ball, Snooker and English Billiards. Steve Davis competed, as did Oliver Ortmann. The winner of both events was Quinten Hann, an Aussie who was a top 16 ranked snooker player and had won the World English 8 Ball title and a 5th place in a World 9 Ball Masters event, if I recall correctly.
He also performed pretty well in the IPT events, despite having a pretty ordinary break.
If a similar event was held today, I'd have my money on Selby over Appleton, by a close margin, assuming that Selby could put a few weeks into US game practice. Though he hasn't played the game much, I'm pretty sure he'd be as good as any US rotation player within a few weeks of practice.
Colin