dropping a stone floor will create a rough stone floor on whatever surface it lands on. ive got little spots of microcline all over the native gold floor of my dining hall thanks to this. collapsing blocks or floors allways land directly below the tile they fall from, but they do spread cavein dust all around where they fall and land, which will move items around, destroy buildings and knock dwarves, pets, and goblin sieges unconcous.
if stone makes a stone floor when it lands it is not unreasonable to assume soil will do the same thing if you drop it down from the surface. also, once light has reached an area you can never make it go away, but i think the indoors tag may stop the grass from growing. no problem there, you allready have a shaft all the way to the surface which you used to get the soil down in the first place, just stick grates or bars over the top. remember though that grates, bars and bridges will not support any constructins, so you are actualy going to need those obsidian pillars you mentioned to hold the roof up. obsidian is very pretty irl, like black opaque glass, sometimes with colourful bands in it.
this is not only going to keep your dwarves clean, happy and sociable, its also going to stop cave adaptation. im thinking ill try and put one of these in my next big fort as well, its an awesome idea. make sure you post the map or at least pictures when your finished, or if you manage to do something amazingly wrong and colapse the whole middle of your fort and flood the now hollow shaft.
some tips - when collapsting things be super extra sure you dont have a hollow under the spot you want to colapse on, or it will go right through. a single falling tile will make a hole right through the middle of your fort if your not careful, and you may not have another soil layer to try again with. make a backup save beforehand. once i accidentaly collapsed something onto a floor with a stack of apartments underneath. dwarves were sleeping in most of them, and their beds were all right above each other in the cavein tile. at the bottom was a high pressure water resovoir. that was an amazing disaster.