I'm testing on a recently acquired C100P (I've got one of most the ARM based Chromebook models at this point) and running into some weirdness.
I did a flash of the armhf to a microSD and then enabled USB boot and rebooted and used Ctrl+U and then chose the expansion installation option and then installed the GUI but after a reboot it appeared frozen. I was able to change to a VTY and stop lightdm (after a couple attempts). The error messages I saw in the console were related to devfreq and panfrost errors complaining about not being able to update frequency transition information.