I got a system working, which I proposed earlier, that combines the
no-power-required magma trench and WanderingKid's
impulse elevator version. The carts only fill with magma if the trench is at 7 units, if it's at 5-6, they often (but not alway, I think) come out empty. And they do come out with very little speed if the trench is full, but haven't had carts get stuck down there yet (I do have a pump to empty&evaporate those 4 tiles, just in case, though).
I've been meaning to do a full writeup, but I was trying to get some other rather more important (non-DF) stuff done first, and I wasn't at home the past couple of days, and stuff like that. Still going to do that writeup, but still have more important non-DF stuff I should do first :S
There's also the fact that there's still some kinks in the system, namely that steel minecarts are heavy, whether filled with magma or not, and I was hoping to have my dwarves haul them around in wheelbarrows once I get the magma to the top station of the track station, but they refuse to haul minecarts in wheelbarrows anywhere except to "place track vehicle". Or haul them to furniture-type:minecart stockpiles without wheelbarrows, either. Don't know what's up with that. And guide orders without tracks in between end up with the carts being carried, as well. So now I'm thinking I might have to make it a multi-cart system if I want to lower the amount of micromanagement while increasing throughput, but I'd probably use something more like the delayed-firing system WanderingKid had in his single-pick challenge videos for the "torpedoes" in the minecart grinder. Minecarts on hatches on top of ramps, triggered by the previous minecart going over a pressure plate.