I have followed the instructions -- downloaded the tvOS beta -- using iTunes 12.3.0.44 I restore using the option key -- iTunes says the restore succeeded and it's time to replug the Apple TV into the TV and disconnect USB.
At this point the device blinks slowly for a while, then starts blinking rapidly, then at some point I see the restore screen again on the TV asking me to reconnect the USB and do a restore.
Rinse, repeat. It appears to fail the restore somehow and just be in an endless loop. I've tried triggering the restore from 2 different macs probably 10 times at this point but to no avail.