Virtualization of OSX: Read-only main VM, delta stored separately?
Regarding the mounted ramdisk (tmpfs), it is possible, see some example:

$ hdid -nomount ram://256000  # 256000 (sector) * 512 (bytes/sector)
/dev/disk1
$ newfs_hfs /dev/disk1
Initialized /dev/rdisk1 as a 125 MB HFS Plus volume
$ mkdir /tmp/space
$ mount -t hfs /dev/disk1 /tmp/space
...
...
$ umount /tmp/space/
$ hdiutil detach /dev/disk1 
"disk1" unmounted.
"disk1" ejected.