Thanks guys, for the speedy replies and suggestions.
------
Brett: Good call - I hadn't looked at that addon, since it didn't come with the Educational download packs for Cadimage. I've since found it as a separate download from Archicadselect.com. I've looked at the manual, and the example they illustrate is a flat property. Do you know if the produced "Envelope" will follow the mesh, no matter how sloping the terrain?
Dwight: I hadn't thought of using beams facing along the property line, not inwards, so thanks for the suggestion, and illustration. It's not quite what I was aiming for, fully - places where the boundary line is straight and the contour of it heads uphill/downhill in a convex shape, the 45degree planes of the two consecutive beams don't meet to form a continuous surface. It will be a good quick technique though, for looking at discrete segments of encroachment at a particular points along the boundary.
Erwin: You may have misinterpreted - the property is actually a rectangular area, not an irregular shape, but the elevations along all boundaries are very irregular, so a polyroof won't be suitable.
----------
I've actually be experimenting with a number of techniques, and I think I've cracked it, both for speed, visual quality and usefulness. I'll post a tutorial (my first!) when I've refined the speed aspect.
So I've solved my larger problem, but the smaller question still remains (for other future circumstances)...
Can I select an irregular polygon fill, and then resize it's overall height without affecting it's overall length? It seems such a simple operation (and common in all other vector apps), that I can't believe it's not possible.