I enjoy experimenting with OSs on my desktop but I have my regular OS ghosted so I can restore it when done trying out another OS. Is there a way to ghost the OS on Nexus 7 before installing a different OS? Or do you have to reinstall the old OS and lose all settings and configurations?
You can do a nandroid backup of your system, that will restore the OS and the apps/settings. In order to do that you need to have a custom recovery installed (or at least booted into).
Alternatively, you can use an app to perform a backup of your apps/data (like My Backup or Helium, if not rooted, to use with it's desktop counterpart), or Titanium Backup (if rooted). Also, download the appropriate system image from Google (the one that corresponds to your hardware model). When done experimenting, you can reinstall that factory image, then restore your apps and data from the backup.
Not complicated, but a bit time consuming.