We answered this question via email on May 24th, but FYI for everyone else: any chassis type other than laptop, notebook and portable can be considered a desktop, so I would recommend adding all chassis types to a dynamic desktop group except for the aforementioned ones. We read the chassis as it is stored in WMI (Windows Management Instrumentation) on the computer itself.
If you build a report under Reports\Create New Report, you can also base it on the tblPortableBattery database table. Windows computers with an entry in this table are laptops. (TblPortablyBattery is not available for filtering in dynamic asset groups.)