It's great that you could start to program this plug-in.
The polyline is a nice way to get some coordinates by reusing existing features of Sweet Home 3D. I would suggest that you request to draw this line in a clone of the current home that will display only walls. You could display this clone in your dialog box with your own plan component / controller, using subclasses of PlanComponent and PlanController classes (VideoPanel class uses for example a subclass of [font=courier new]PlanComponent[/font]). If you're interested by this, just ask for more details.