I don't think there are 3D line type options (unless that snuck into v10 without my noticing).
Two ways I can think of:
1. Use a transparent material with a fill pattern (dots for grey or dashed diagonal lines come to mind) to indicate the demolished parts. This requires that transparency is on in the elevations so you have to set your glass material to 49% transmittance if you don't want to see through them (it still renders fairly well this way).
3. Make views of the elevations with just the demo layers on, convert to drawings and make the lines dashed. These can then be overlaid on the main elevation views in the layouts. You will have to repeat the change to dashed lines each time you update the demo elevation views, but that shouldn't be very often (if at all) unless the scope of demo keeps changing.