Turned out to be quite easy, as long as you have a firmware <3.55 to start with. It is also recommended that the PS3 doesn't have access to the Internet.
- Download the original 3.55 firmware from a trusted source
- Copy it on a USB stick formatted in FAT32 under a folder tree called PS3/UPDATE
- Start the PS3 with the stick in, choose firmware update, update from removable media
- While it is upgrading, download a custom 3.55 firmware such as the Kmeaw one from a trusted source
- Stop the PS3
- Copy the custom firmware on the very same USB stick, overwrite the original file
- Start the PS3 in Recovery mode. To do so:
- Press and hold the Power button until the PS3 starts then stops 15 seconds later approximately
- Press and hold the Power button until you hear a double beep
- From the recovery mode, select upgrade firmware
- Put your USB stick in and go ahead!
1 From Cèbe -
It is then possible to run backups, using GAIA manager for instance.
I found this tool to be a bit picky when it comes to backups:
* They should be placed in a FAT32 formatted disk under a GAMES folder.
* FIX 3.55 files should be put under the USRDIR of each game.