Even in developer mode, ChromeOS provides a very limited set of tools, no package manager, and a mostly undocumented environment. It's not really appropriate to use as the base of a developer machine. Ubuntu, on the other hand, has all of the right things to make it easy to use and extensible for devs.
I think that in long term it will take much less effort to customize ChromeOS than Ubuntu. Initially it will be harder but after that it will require next to no maintenance. Changing the kernel sounds much more complicated than finding a way to install a package manager. Google might also be willing to participate in this in more ways than just helping set up the environment, which could be very beneficial for the whole project.