Add a checkbox to the Import Options to import all geometry at zero elevation (2d)
Adding to this, the ability to import everything at Layer 0 (or Layer 99) would be useful, too.
This is following on by a suggestion from Anssi to do this:
A simple “quick and dirty” way to “flatten” an AutoCad DWG is to import it into SketchUp, select top view, switch camera to parallel projection and export it to DWG as a 2D image, to full scale. You can then re-import the result into a new SketchUp file.