Two mechanisms for this - one which is relatively simple, and another which is more complicated yet would probably be far more impressive. So, in an attempt to walk before I run, here's the simple version first.
Imagine a fairly ordinary looking organ (no sniggering at the back, please), with a
range of tin funnels arranged on the ground in front of it in a semi-circle. There is one funnel for each key on the keyboard, and the funnels themselves range from cup-sized to bucket sized, graduating in size as they proceed around the arc. Still, each funnel is exactly the same distance away from the centre of the organ, and there is exactly the same distance between the centre-point of each funnel, no matter what size it is.
But - aye; now's the rub. I'm sure we've all seen those dancing fountains, where water is pumped in a precisely controlled jet. Each jet of water arcs through the air in a single stream, and (through the strategic positioning of separate pumps) seems to leap from pool to pool balletically, much to the assorted "oohs" and "aahs" of onlookers. I'll post a link with some pictures if I can find them.
So - and I'm hoping you're ahead of me already - install such a controlled fountain-jet into the top of the organ. Instead of some throaty bellows wheezing to life when you tickle the ivories, now a lithe stream of water is let loose. Each key alters the rotational direction of the water nozzle so that the flow is always directed into the appropriate funnel. The funnels, of course, are all pitched perfectly so that when the water arcs into them they each give the correct musical tinny tinkle. The water siphons back from the funnels to the pool that feeds the pump that drives the water that plays the tune. And so on.
Obviously, there are drawbacks to this setup. For one - the performer is always playing the tune a couple of seconds or so before the audience actually hears it. It would be very difficult to keep ahead of a piece of music when you're hearing notes you played a little while ago. I suppose the player could wear headphones, and listen to a Bontempi-type rendition of what his fingers are currently doing, but, to me at least, that would get away from the spirit of the idea. I'd like the performer to smugly smile to himself as he gets the audience reaction to what he was playing a few moments ago, and silently think to himself, "just wait till you hear/see *this* little arpeggio..." Also, playing a single note for any length of time might make the funnel fill faster than it can drain, which would result in a rising of pitch. And perhaps lend a sense of urgency to even the dullest of tunes.
But by far the most crippling setback is the fact that, in this basic setup, you can only play one note at a time. Fair enough, the above would be (relatively) easy to construct, and it did make explaining it a little easier, but it's obviously not using the full potential of the idea. Throw all expense to the wind, allow a separate water-jet for each key, and let the insanity flood on...
Not only do chords unleash impressive plumes, but suddenly the funnels don't have to be as regimented as they were before. With each stream being autonomous, all they have to do is not collide with any other stream. So generous spacing between funnels is probably quite useful. All the squirts have to share is a common time-measure - it just has to take them all the same amount of time to reach their acoustic funnel for the tune to flow on as played. Extend the time between playing and hearing, and you could have a truly spectacular display. I'm imagining funnels actually situated in the audience now, with streams describing a high, spectacular arc as they reveal their note only upon splashdown right beside you. Bass notes could be situated far behind the crowd, and the organ could unleash a riot-bashing direct water-cannon type-stream in order to sound them. It would take perhaps many months of trial and (computer-simulated) error to work out the optimum placing of audience, organ and funnels, but it should be (at least theoretically) doable.
Imagine the sense of tension that would build up during a particularly tender passage in the music. The water jumps like a frolicking salmon from tiny thimble to slightly-larger thimble: still, the audience knows that an exciting crescendo is brewing. Suddenly thick worms of water blast over your head, and before you even hear it, you know something is about to happen...
And now, if you'll excuse me, I suddenly have to go pee.