Still not 100% working but to get a first impression:
I created a task with the attached patches here: https://phabricator.whonix.org/T714
I tested it with the 126.96.36.199.1-developers-only sources. It works currently only for the raw gateway. Didn’t test it for virtualbox or qcow2 but there should be no differences. I could not build the workstation but this is not related to live mode. Some packages (like onionshare) were missing and so the build failed generally. Since the build uses dracut packages from testing but debootstrap uses only the ones from stretch you have to put those packages in the /etc/debootstrap/extrapackages folder before you start building. The required packages from testing are dracut, dracut-config-generic, dracut-config-rescue, dracut-core, dracut-network, iputils-arping (required for dracut-network).
After this you should have a working gw image. For the first boot I needed to change the root to /dev/vda1 in grub but maybe this is just a problem on my side since I build in a chroot. After the first boot in non-live mode issue “update-grub” then the two different live modes should work.
The patch-package atm probably corresponds not to the official debian standards