Works great - but not without a 'heart attack moment' first time around.
I pushed the kfhd8-amazon...zip to the /sdcard/download folder from a Windows box. It was put into /sdcard/0/download.
Once in TWRP, I naviguessed to the /download folder to install the old software - and the folder was blank (apparently TWRP defaulted to the /sdcard/download folder). Ok...Hashcode warned us about the duplicate folders in JellyBean, so I backed out of 'install' and used to file manager to find the kfhd8-ama...zip in the /sdcard/0/download folder. It was an easy click to select it to copy, then navigate to the 'other' download folder, then press paste. FAILED
I tried again and that copy failed as well.
Navigating back to the 'install' menu refreshed the screen and the update software zip was magically there. Maybe the failed copy didn't really fail? Maybe the initial read of the /download folder failed and needed to be refreshed? Dunno.
But the flash worked perfectly. And it's apparently resilient enough to catch newb errors - I was so happy that the software appeared that I forgot to wipe the system again after installation. Pressing reboot to the system resulted in an attempted boot and a return to TWRP where I could give the system a good wipe. The next boot was normal.