In general, you should place a modest bet on the flop.
I think optimal play would be to get the action started right away by betting the flop modestly. Too hard and they'll fold, too small and you're not building the pot enough and could be seen as slowplaying. Your opponents will figure that most of the time no one will have the nine anyway, and in the rare instance that someone does, that player will usually just check it... so a modest bet probably won't be taken too seriously and may, in fact, make them put you off of holding the nine. You should get a fair amount of callers this way because everyone has a tempting draw to a Full House no matter what their hole cards are, whereas anyone who folds to your bet probably figured in advance that they wouldn't be willing to stay in under any pressure regardless of whether they paired on later streets, thus making it better in the long run to eliminate them from the hand immediately (so as to encourage further action on later streets). So, I'd just play it cool and make it appear that I'd flopped a weak Full House with a pocket pair, or was on a draw with one or two good facecards (which will be half of the table anyway), and then continue to maintain that image on fourth street. By the turn you'll have built a pretty good pot and those who stuck around will be priced in to chase the river whether they've made their hand or not. By the river a few should have made their Full Houses. If the turn and/or river come high cards, you might get seriously paid off. But if all you do is check the flop, you're not building the pot and you're actually more likely to get put on slowplaying the nine, I believe, than if you simply represented a weak Full House, or a draw to a strong one, with a modest-sized bet.
This is an excellent question that warrants further study, perhaps for the development of a system. This flop (which occurs only .01% of the time, or once in ten thousand flops) is one of the rarest and most potentially profitable in the game. In fact, if you consider how much potential this flop gives to so many other players to improve to very strong hands, coupled with how virtually guaranteed you are to win, flopping quads with one of your hole cards might very well be the most theoretically profitable flop in the game.