You may want to take a look at the sample:
\Revit 2009 SDK\Samples\VisibilityControl\CS
It may provide the methods to hide a specific element.
And the sample:
\Revit 2009 SDK\Samples\ImportExportDWG\CS
May provide the necessary methods to export to .dwg
I haven't looked at these myself to know the specifics, but it may get you very close.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.