First attempt failed tonight. None of my embark civilizations seemed to have either kind of coal boulder, so I drug along a bunch of coke and iron to a volcano.
I built a simple self resetting linear accelerator, fired by a dwarf push. Empty test run showed gun functioning. Filled cart half full of coke. It shotgunned the coke perfectly and reset. So I mined the last diagonal tile needed to fill the sump with Magma. Minecart filled with coke entered three tiles of 7/7 magma before continuing onward. It passed through the magma at about one tile per tick. It did not skip over the magma, as I watched tick by tick and saw it on each of the three magma tiles.
Coke was not ignited, and shotgunned the target zone just like in the test run. I began to wonder if maybe coke wouldn't light with magma. So I let the cannon fire a second cycle empty. I figured this would pass the empty minecart under the magma, fill it with magma, throw the magma into the trench filled with coke, and ignite it. No such luck.
The mine cart passed through all three tiles of the magma sump without filling. No magma entered the minecart, and no gob of magma was thrown.
Maybe a longer dwell time is needed? If so, a longer sump trench might work. Maybe the mine cart needs to pass through slower? If so, removing a couple impulse ramps might allow it to fill with magma / ignite contents. So far "Plan A" is a bust.
As for throwing "Lignite Fusion Bins", my understanding is on impact with ground or wall will cause a bin/cage/container to spill its contents like a grenade. Aka, a cage with 100 cats would fly straight, impact, and scatter cats with forward momentum. If that is so, the lignite bin would separate from each other.