I come bearing terrible news, my friends. I have carved out (as in dug around) a stone pillar, 5x5 and fiftysomething Z-levels high, suspended by a single floor tile in midair just above the surface of the great magma sea. This time, there was no dwarf inside it as it fell for two reasons - I had no access to water and I have already tested that the fall can be survived). My hypothesis was that it, being a natural wall, wouldn't melt and so would just sink several Z-levels until it reached the semi-molten rock (which I knew was there) and stay there. In case of failure, I expected that it would melt level by level until it disappears into the magma. What happened, though, completely defied my expectations. A miner channeled out the one tile holding the entire thing in place. The pillar fell the one z-level. It touched the magma and... the bottom half of it evaporated. E-VAPORATED. The upper half (not exactly probably, I don't really know) turned into 7/7 squares of magma, which then proceeded to fall down the now empty shaft and increase the level of magma.
Now, while this obviously makes any further attempts at making a magmasub futile, it creates a lot of new options, because it is essentially a very simple, if non-renewable, way to move magma fast to upper levels of your fortress, eliminating the need to use pumps.