[jlgaddis]

> I'd like to figure out how to do this with VM disks as well but I've never had a pressing need.

Everything that you will (probably) need is here: http://libguestfs.org

If you happen to need something that isn't already included, the APIs make it pretty easy to build your own tools as well.

Conveniently, those tools are also already packaged up for your distro (unless you use some esoteric distribution, perhaps).