It sounds plausible, but you couldn't keep reusing the same tarball, because it would have to be kept upgraded and rearchived. You could use squashfs as an alternative to tar, but I expect the frequent unpacking, upgrading and repacking would become just as tedious as any other method. You would also have to ensure it had the right modules for your running kernel, as unlike a VM, it wouldn't have its own kernel. Would have to be the same arch as your main system too, so you'd still need a machine of the other arch.