Also, the question is moot. In the real world, what we can do is limited by physics. In Dwarf Fortress, Dwarven Physics is limited to what the game engine (and thereby the player) can do.
Not quite true. There are Dwarven Physics restrictions, but there are dwarf movement and activity restrictions, and these two don't necessarily match.
Suppose it was impossible to drain an ocean. In this case, a stone column protruding from the ocean's floor would be perfectly legal according by Dwarven Physics, but it would be impossible to build it using dwarf labour.
P. S. On an afterthought, though, it would be perfectly possible by magma-casting it up to the surface-level and then building normally from there. Heh.
To put it another way, the simulation can't factor in variables for which it has no information - if gravity is turned off in the INIT file, gravity is no longer a part of Dwarven Physics. In the real world we can't go mess around with our raws and turn our household pets into thermonuclear weapons (which is probably for the best), but by the standards of Dwarf Physics, cats spontaneously radiating the head of a thousand suns is perfectly acceptable for the simple reason that the programming says they do.
Tweaking the raws has nothing to do with the question. Tweaked raws create their own universe with it's own version of Dwarven Physics.
Personally, I feel the "impossible to build but physically legal" constructions can be designed using various movement-restricted locations, like aquifers or magma pipes.