[Feature request] loadvm snapshot as read-only

There are many ways to take and manage snapshots in QEMU, but one main feature that's missing is the ability to 'loadvm' a LIVE snapshot and have all future changes redirected to a temporary file.  This would effectively be combining the -loadvm and -snapshot switches and make the snapshot read-only.  With this feature, users would be provided a "sandbox" and be able to start and restart the same live snapshot without corrupting the image in doing so.

I found a lot of discussion about this topic on the mailing list years ago, including some patch submissions, but none of the conversations panned out.

http://lists.gnu.org/archive/html/qemu-discuss/2011-10/msg00011.html
http://copilotco.com/mail-archives/qemu.2008/msg00072.html
http://web.archiveorange.com/archive/v/1XS1vcusGInZKG2e0ImX
http://marc.info/?l=qemu-devel&m=117191084713590

What would it take for this feature to be added, and can we use the patches submitted by Eddie Kohler to enable this feature?