You can do it to an external drive, so why not a flash drive? Should work fine to create a ghost image on one, as long as you can see the flash drive when you boot.
You can Ghost from a flash drive. You need to make your flash drive bootable. I copy the ghost.exe file and the image files to the drive and run it straight from the drive. It doesn't seem to be a quick as ghosting via the network but if you're at a remote location it's a solid alternative.