Look at page 16 posted by me, it is confirmed that US Region 1 (BD-D5700) WORKED...!!! Just follow my instructions and you'll be fine.I have a US (Region 1) BD-D5700ZA running 1028 firmware (6/21/2012). I would like to guinea pig for the downgrade.
Like I said, I can confirm that US Region 1 (BD-D5700) firmware 1028 downgraded to 1015 successfully. That being said, it should work for all Region 1 D5700, including 5700ZA. You just need to keep trying. One more thing, make sure after downgraded to "1015", turn OFF update via "Standby Mode",
(Settings, Support, Software Upgrade, Download in Standby Mode: OFF).