You can make a usb/disk image and then test drive that. When you rebuild the next image you can also choose to use additional formats, including .iso files. So you can modify files using the usb image and then rebuild the image as an .iso. The same also works in the opposite direction. If you have an .iso built, you can use the additional formats option to build a usb/disk image to modify the files and then rebuild and use the .iso format again. It's actually pretty flexible. The only downside is waiting for the appliance to build every time, but it can be done.