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Try putting a hatch on your magma chute and keep it locked when you're not dumping stuff down it.
I tried that, and it seems that if you are dumping many things at once (which is usually how it goes) groups of haulers will hold it open long enough to get spat at. I could fix that with a second hatch underneath that's lever operated, and items could periodically be dropped into the magma sea from there, but the dump hatch was located in the magma foundry, which was directly above the magma sea. Also, I forgot to set up the plumbing shafts to be under the solid tile of the workshops, so the whole thing was a death trap, considering the half dozen magma crabs directly under the openings. My metalcrafter/mechanic got his arm blasted off above the elbow, and died of his wounds because I had no medical system yet, and no one would even move him off the floor because they were all busy dumping the boatload of stone that was blocking the planned bar/block, stepping over his rotting alive body and charging headfirst into enemy machine gun fire to dispose of the gabbro. I suppose maybe I shouldn't try to set up a haphazardly designated magma foundry in early summer of the first year.
The solution to your hatch problem! Two hatches, a gear, and a pressure plate!
The pressure plate is located at or immediately adjacent to the dump point, so that when a dwarf is on the plate, the hatch opens allowing the normal "oh, i guess i should dump it into this hole!" to happen. At the same time, the pressure plate is connected to the gear, and the gear to the second hatch, such that when the top (first) hatch opens, the second one closes. This should lead to open-top-close-bottom, dump in items onto lower hatch, close-top-open-bottom and everything falls down to where ever!
Oh, and the advantage of a larger section of clay being dropped, assuming it doesn't auto-transmogrify to something non-clay like, is that only a single clay getter can get clay per tile at any one time. And since a good potter can use the clay way faster than his accomplice can dig it up ...