On Land

Environment Information
At Rill Architects we run ArchiCAD on Mac OS X. If you work at Rill, this is your stuff. If you don't, but you work in ArchiCAD, you may find something interesting. Anybody else, I don't know.
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Irregular polygon object 1
Slabs are easy.

Irregular polygon object 2
With the polygon editing palette, irregular slabs are easy.

Irregular polygon object 3
For complex section geometry such as a corrugated sheet, you need an object. But such an object is usually rectangular. There is no polygon editing for object elements. It is difficult to code even pseudo-polygon editing into an object, and still the palette wouldn't be available. More to the point, AC library objects such as corrugated sheets certainly do not have this ability.

Irregular polygon object 4
So place a slab and the object. Make the object extensive enough to cover the whole slab. Put the slab on an SEO operator layer. Perform an Intersection solid operation with the object as target and slab as operator.

Irregular polygon object 5
Then you have this.

Irregular polygon object 6
Then you can edit the slab like a slab.

Irregular polygon object 7
And the intersection is updated.