Hmm, had success on a shiny, new 3C*** serial, but my older 36*** won't root.
It just sits forever at a black screen. I have a Teensy++ and have tried both the plusplus_16664.hex boot code that worked for my 3C*** serial and the plusplus_12940.hex version. Both Chromecasts were on the same Google OTA build. Is it possible this exploit doesn't work on the 36*** serials?
I can't SSH to it, neither during the blank screen (which I let sit for 20+ minutes) nor upon rebooting (no root), so I can't give you the flashcast.log file, sorry.