good eye on the exploit, Starver
An eye, only. I've never actually done it (knowingly, at least), and not even classed it in my mind as an exploit, merely an oddity of the DF universe.
But I was thinking, inbetween my last post and this:
Based upon the standard layout (I think) of each minable block in a single Z-level being 1 unit floor and 7 units space (as defined by the liquid measure), mining should produce 7 units of rock per square. Cutting a channel would produce the lower level's 7 units and
this level's single floor unit. Channelling a floor already over a void would produce the floor unit only. (For now let's ignore
where this rock will 'go'.)
As for building, a pillar[1]/wall segment (upon a floor) should need 7 units of rock. Building a pillar/wall over a chasm would involve the eighth block. Laying a floor on an open space involvesa single unit. Laying a floor where a floor already exists (above a void or not) uses a single unit of new material, but brings up a single unit of the original material.
Trees may provide a different proportion of wood material (not filling the entire 'unit square', except for massive 'redwoods', but of course all but the youngest fellable trees extending into the Z-level(s) above) and may be used less 'densely' (hollow partition-style walls, floorboards with voids below, and treatments for the lower level ceiling as required... possibly a different allowance for vermin movement).
Workshops could have wooden legs on their workstations with stone worktops, to pick and choose as materials are available (or required, for certain workshop tasks) and a chair would use less wood than a wooden table (a stone table may need more stone, for integrity's sake, and may need two haulers to jockey into position). Earrings and other small items would need minute amounts of material (many more multiples per unit stone than DF currently gives for such as mugs, etc, and/or create a large amount of sub-unit waste that needs spoil-tip hauling (useful as aggregate in wall and road building, as well as other purposes?).
Note that this is not a DF developmental change that I'd advocate, because it may be too great a leap for existing players to absorb without problems.
I'm also sure that the above mechanics/physics have been discussed to death, even without searching the boards.
[1] Given you can squeeze between diagonally-adjacent pillars, there's additional adjustments that should perhaps be made to account for the difference between 1x1 cross-sectional area of a full wall and pi*0.25 cross-sectional area of a round pillar, e.g. 6 rock units for a pillar, but when orthogonally adjoined to another pillar, wall, wall end or natural and undug rockface, another unit is needed to perfect the seal. The maths still isn't perfect but could be fudged, or made better by using 64 unit divisions/etc and taking into account all wall faces, and/or assuming that even wall doesn't take up a whole cross-section, but recessed on all sides that don't abutt with other walls.
Effectively taking up the square, but allowing treatment of the wall as two-sided where you can attach decorative features (paintings, arasses, stone alcoves/relief murals) on one side or the other of the partition within the space between edge of 'square' and the wall.