After upgrading from Windows 7 to Windows 8, is it possible to "restore" to Windows 7 by running system restore in Windows 8 and selecting a restore point prior to upgrading?
No. There are dozens of reasons why this couldn't possibly work.
This is just completely out of scope for the job system restore is designed to do. It would require keeping an entire image of every OS file and everything dependent on OS files on the disk. It would require preserving things like application executables and libraries. It would require some way to connect snapshots taken from one OS with snapshots taken from another.
This really is like trying to use a jackhammer to get demons out of someone's head. There's just no connection whatsoever between what the tool is designed to do and the effect you are trying to get. It just makes no sense whatsoever -- a total non-starter.