Aside from the Moon game linked to from the Domino Plaza, JAred D. West mailed a variation. It's the way he has been playing it all his life:

There is a moon variation with 4 players. Most 42 rules apply aside from those concerning count dominoes.

Bidding is from 4 to 7 as in moon but you do not take out the blanks, and you don't count tens and fives in scoring.