The Embedded edition of Windows (e.g. Windows Embedded Standard 7) has a feature called Enhanced Write Filter that pretty much does what you want. It creates overlays for logical disks and discards the overlays on reboot. There's also the option of committing changes to disk (for servicing etc).
I believe that EWF will be carried over to Embedded 8.
That said, licensing issues may prevent you from using the solution. Depends on your specific use scenario.