I just tested your idea with grml and it seems to work fine.
The only caveat is that the filesystem is not directly in the iso image, but it's further compressed in a squashfs file.
So, you need to
mount grml64-full_2014.11.iso /mnt/tmp
unsquashfs -d /bedrock/strata/grml-full_2014.11 /mnt/tmp/live/grml64-full/grml64-full.squashfs
I bet that most other live images work in the same way.
"unsquashfs" is in the "squashfs-tools" package on Gentoo and Debian (just tested in grml from Bedrock
), it probably has the same or similar name in other distros.