I tried messing with this tutorial, but am not sure I made any progress.
My Brain Hurts: Porting Clockwork Recovery to New Devices
Used it to build an .img file, but it's only 5MB so I don't know if it's even worth testing. It was built from the contents of my phone's boot partition so maybe it should be smaller. I don't know.
Snake - It's just that extract-files.sh script that takes proprietary vendor files off your phone. I'm not sure if the make process kicks off that script or not so I just run it manually prior to the make process. (just in case)
I agree it would be really helpful to understand the big picture in all this. Hopefully someone out there with more experience is working on this stuff too.