After hours of reading and multiple false starts I was able to disable the Reverse beep using my Android phone! Ha!
Using one of the ELM-327 knockoffs from Amazon.com (2 day free Prime shipping) and 'ELM 327 Terminal' from the android store, it was a 2 minute job (that took me 20 hours to figure out).
I did all this with the 'Ready' light on and the doors all closed. Not sure if that is needed, but just in case you can't reproduce my results.
After the terminal program is all connected I just typed the following commands into the little white box on the bottom left of the terminal program (not clear that this is where you are supposed to type and not very big, but it works).
Turn 'Timer' (right side of screen above 'clear') off (or commands repeat over and over again).
Commands:
AT SH 7c0 (hit 'Send')
Returns: OK
21ac (hit 'Send')
Returns: 61 AC 00 (meaning continuous beep)
3bac40 (hit 'Send')
Returns: 7F 3B 78 (no idea 
Returns: 7BAC (meaning done)
21ac (hit 'Send')
Returns: 61 AC 40 (meaning now single beep)
I then popped the car into 'Reverse' and joy! joy! only 1 beep.
Read more: http://priuschat.com/threads/turn-off-the-damn-backup-beep.101145/page-2#ixzz3C1wEDqsI