Well, it is probably possible to modify the filesystem drivers, to read at a certian offset, however, I dont beleive anyone has looked into that yet. (Currently easier to just transfer data from the PS3 Linux over a network!)
Furthermore, there are two 128mb Samsung flash chips (iirc) on the PS3's motherboard, Dont know if ones for PS3, other for PS2, or if its a backup (not a clue!).
What the PS3 does is on bootup, it does some stuff, initalizes the hypervisor etc, then checks a flag set in the 4mb user writable "otheros" area, a 0 or 1, for game os , or linux.
If its the GAMEOS, well, it does some more stuff, loads the XMB up, checks for a game/movie disk, etc.
If it is the Otheros, it uses its bootloader (aka part of otheros.self) to load into the user provided linux files, eventually booting linux.
Its basically a two stage bootloader, the OTHEROS.SELF boots the OTHEROS.BLD, which then boots up a full linux kernel!