Interesting process you're going through there.... I am doing something similar but at a slow pace due to many other priorities. I'd be interested in other off-line collaboration, to the extent you are willing/able.
As for your system builds, wouldn't you be able to put something at the bottom/base of your %post section, or the script(s) run from it, that can 'signal' back to your control center that 'hey, I'm done over here!'?? Similar to the email concept, but different implementation.
I think what is cogent here is the definition of 'done'. What does it mean for a system/VM to have reached that point. So, in other words, put a tangible action-point there, that gives you a viable indication so other (I'm assuming) activities outside of your VM-build operation can exfiltrate data/report(s) and destroy the VM, etc...
well koan can monitor for when a VM is finished building and then restart that VM so perhaps you could make it do some other form of signal to indicate that its been rebooted.
Or maybe use firstboot ??