I did exactly that: just unpacked the navcore(s) on the wiped TomTom drive and boot it. There was no error message, the unit just kept rebooting. With some navcores the most I could see was a splash screen and boot loop, but with some other navcores there was nothing at all. They were frozen and I had to press the reset button to continue trying.
As regards to the wanted tutorial, I meant I could unpack the original kernel image and inject some files, maybe editing some configuration files, and repack. The navcore is a Linux operating system, isn't it? I think I would replace the app that is reponsible for handling the map but use the existing kernel driver(s) from old image. Is that the way people have made the custom outsourced navcores?
Certainly I have willing to try more but rather than just try them blindly I'd like to know what the specialty of each navcore is. For example one navcore may have driver for a certain GPS chipset while another navcore supports Bluetooth, etc.