I was exactly the same. Search for manager2.pkg and dump this onto a fat32 USB stick, or HDD. Let your PS3 boot fully, then insert the USB device into a spare USB port. (USB device must be inserted after PS3 is running and logged on)
You should now be able to install backup manager. Using backup manager, backup your game. (takes about an hour)
To play, you need to do the same again..
Boot PS3, allow to come up, (using Rockbox of course).
Insert USB device in spare USB port, run backup manager, find your game and choose run.
Allow the PS3 to drop back to the dashboard, open the other folder you can now run the game from there.
And wait for the PS3 to fully boot up, before you insert the flash drive.