About the first issue you reported; I think I know why you got stuck on the password prompt while generating an image. The behavior seems to be due to delay in arrival of the 'New Password' prompt. I had to increase the timeout in rootpass.exp to 4 and this value might require some tweaking.
Got rid of the expect script and replaced it with a simple 'chpasswd'. I've tested image generation in CentOS on Cubietruck as well as F21 in Qemu. Thanks for your feedback.
Regards Mandar Joshi