I have discovered a new way (new to me at least) to show foundations as hidden lines in elevation views using the new "show hidden lines" tool in 9.0. See the attached image for an example of the results.
Methodology:
1. In the elevation view create a filled region with invisible lines for the edges, no pattern, a transparent background, and white color. In order to make it easier to select the foundation walls just create a small rectangular filled region for now that leaves the walls to be shown exposed.
2. Select the "show hidden lines" tool.
3. Select the "multiple selection" option.
4. Select the filled region as the object to allow items to be shown through.
5. Select all of the foundation walls that you want to be shown as hidden lines. NOTE: They do not have to be behind the filled region at the time of selection. They will not change to the hidden line style immediately but don't worry. Exit the "show hidden lines" command.
6. Select the filled region and drag the extents to cover the foundation walls using the shape handles. Revit will remember which walls you selected to be hidden and show them that way once they are covered.