I found the distance from the magma sea up to my sand deposits a bit overlong, so I created a nether-cap minecart to carry some up to the sand. However, as soon as the cart followed the tracks into the magma to scope some up the cart vanished without a ripple. Checking the wiki, I saw nether-cap "dumped" into magma is destroyed, so apparently driving (actually pushed) through magma along tracks is considered "dumping". It seems I will have to wait until the goblins come over and donate some iron (I don't have any metals in my embark, and I doubt it would be possible to harvest a couple of hundred nether-cap logs for a pump stack without waiting for several regrowths of the nether-cap population, but I guess it might be possible to use two pumps to pump magma two levels up, deconstruct and move the lower one, pump once more... repeat ad nauseam [but even that would require another woodcutting expedition into the dangerous 3:rd cavern]).
The question part of this post is: how much magma does a mine cart scope up when driven through magma? The wiki states the capacity of a mine cart is 833 "units", but I've failed to find the relation between "units" and 7:ths of liquid depth, nor do I know how much the cart would actually pick up, which I would guess would depend on the depth of the liquid driven through. If the up to 10 units (?) of water a bucket can hold which will fill 1/7 is an indication, a mine cart ought to be capable of carrying 83.3 levels of liquid, i.e. basically fill up a 3*4 pool, which seems a bit much (although in dwarven physics, 1/7 of water freezes into ice that melts into 7/7, so it wouldn't be impossible). Obviously, I'm not too keen on dumping the mine cart contents into a single tile hole only to see most of it flood the area, destroying workshops and whatnot (as well as the hauling dwarf). Hm, would a nether-cap that somehow was loaded with magma be considered "dumped" if it dumped its contents and was caught in the spill?