It appears that you can still download old firmware files from OPPO for US version players. The latest firmware that supported .ISO was December 2011. You can download it from this OPPO page. Just scroll down to the instructions for the thumb drive version and follow those instructions.
There is a complete list of US firmware versions with links to OPPO for each one in post number one of this thread. Keep in mind that at some point it became impossible to back up to an earlier firmware.
Edited by htwaits - 7/5/13 at 12:52pm