Some more tweaking and fancy math (3d intersection of a plane and line) and the shadow tracing sketch in Processing seems to be working perfectly. Shapes are drawn and capped, the file is then exported as DXF and loaded into Rhino to be scaled and exported for 3d printing. Next (and hopefully final) step is to set up a control interface so the model can be manipulated (scale, turn on/off floorplane, etc) and exported on command.